I used a standard spray bottle to get the diameter I wanted. The bottle is about 3 1/8" diameter, then with the wire, I ended up with an inside diameter of 3 1/2".
I started at one end, wrapped the wire around the bottle and zip tied that part, moved the bottle down and repeated the zip ties until I had a wire tube. I used a broom handle to push the bottle through.
Then I pushed the bottle through again as I duct taped it.
My whole tube with sections duct taped.
Taped the tubing in place to make my first cut for a bend. Notice the hood is raised just enough so I can bend the tubing around it. That way when you raise the hood, it will not hit the snorkle.
I cut it along the bottom so I could achieve the desired bend I needed. After I made each cut, I used a small piece of wire mesh to fill in the gap, securing it with small zip ties, or just using the ends of it to kinda weave it in place.
You can see where the hood should miss it now. I actually glassed over my antenna hole after I removed it. I now use a power antenna that goes across the inside of my windshield.
Next two pics show another cut along with the foot I molded and circled the spot where I will drill the hole for the duct work.
Starting to add more duct tape now to hold my bends in place.
Once I had all my bends in place, along with where the foot should go, I duct taped it all. I applied two to three layers of duct tape to the entire wire mesh. Then I started to lay fiberglass all over it. I bought the packages of glass and cut a lot of strips of different sizes and lengths. Then mixed the resin and started to lay the glass. I would lay it long ways, then come back and lay it cross ways. I layed the glass strips all different ways. I would read up on how to lay glass if you never have done it.
Letting the glass cure. After it cures, you need to lay more glass in different directions to start building up the thickness of the snorkle.
I stopped here as that is where I will cut it off at later on.
I then duct taped the fender real good. Then placed the snorkle on it and traced around the foot.So when I lay the glass, I'll know where the foot goes.
After laying the glass, you can see the foot outline still.
I used a long screwdriver to start prying out the inside. You can see the thickness I was talking about earlier. Just shove the screwdriver in there and seperate the glass from the duct tape. It takes some work to get it all out.
This smaller piece I made is for later, to glass on top of the snorkle. This is where the snorkle head will slide down on. I took some big pliers and grabbed to wire, then twisted it until it broke loose and came out.
I had cut the foot off the main tube as seen above. This pic shows the main tube, then the bottom of the foot, then the top of the snorkle.
Twisted and twisted the inside wire and duct tape. 
Then started pulling it all out.
Out it came. It can be tough to remove it, but just work at it and take your time.
This shows where I had cut the main tube and foot apart to help remove the insides.
Here is the glass that I molded to the fender. It will be the bottom of the foot in the next pic.
I cut the top of the snorkle downward, creating a slit. Then slid the small piece into it and used a hose clamp to hold it in place to glass it together.
All three pieces glassed back together. BEFORE doing this, you need to tape the snorkle back in place on the Durango. This will insure you glass it together how the foot originally fit. Once I had it taped back on the Durango, I used a marker and made arrows on the two pieces pointing at each other. Then when you remove it for glassing, just line up the arrows.
I applied Bondo Glass all over the tube. This is where you start smoothing it out. I did sections of it at a time and not the whole thing all at once. Once the glass starts curing, you can use a rough file to knock down the high parts and start molding it. Then apply more glass, continuing to mold it until the whole tube is done.
This is after a few layers of Bondo Glass and some sanding with 60 grit. You can now see where I will start filling it in and smoothing it more.
This is where you start working in getting it smooth. Apply small layers
Smoothing stage. I also used lightweight bondo to start getting a final layer for smoothing out.
I had put some duct tape around this part and layered up to it. Peeled off the duct tape and used a file and sandpaper to get an edge. The snorkle head will sit on that edge.
You can see also where I am working on the foot a little when I have left over Bondo Glass. Getting the foot smooth will come later once the bottom is on it.
Here is the air filter box.
Laying the Bondo Glass to shape and smooth it.
Built up this end, it is where the lid will bolt to.
Here is the lid. I glassed it, then bondo glasses it before removing the cardboard and duct tape.
Starting to take shape
I used two pieces of cardboard and taped them to both sides of the lid. You can see the pencil outline on how big the lid is. I used this method to build up the bondo glass around the lid. Once it cured, I removed the tape and filled in the gaps.
Shows old and new thickness of the lid.
About to drill the hole where the flex hose will attach.
I always put things in place several times to mock it up and make sure it all fits.
Bent brackets from a piece of aluminum that will hold the air box in place.
There are bolts in place from when I ran my stock horns that I attached the air box to.
Hole is drilled, the part that will bolt here and hold the flex hose is coming up.
This piece of rubber was when I ran my other cold air intake box. I cut and fabricated a piece of aluminum and this piece is what the tubing from the throttle body will go into.
Added this brackets from Lowes to clamp down on the lid in the next three pictures.
Once the aluminum brackets were mocked up and done, I shot them with Self Etching Primer.
This is where the flex hose will attach coming in from the snorkle.
This is where the tube coming in from the throttle body. I ran a bead of the polyurethane around the hole, then screwed the piece down after pre-drilling holes.
I also ran beads of polyurethane under each bracket, then once I screwed them in place, I used the wipes from below to clean up the exccess.
The main reason to use the polyurethane is that once it cures, none of the screws will come loose, nor will the brackets or top piece.
Mocking the snorkle back up to glass the base to the foot.
Once I had it all mocked up, I taped the base down to the fender and removed the snorkle. The with the base in place, I drilled all the holes I needed through the base and fender as seen in the next picture. 
This way when I place the finished snorkle back on, all holes will line up perfect. I drilled holes for the bolts that were going to hold the snorkle down. My intention was to reach into the fender through the holes from behind and tighten down the nuts. That did not work, so you will see plan B in a minute.
Bondo glass in the bolt heads to keep them form spinning later on.
Another picture of mocking it up.
This shows the nuts and bolts from behind. All I was going to do is remove the nuts, the bolts woud have been held in place by the bondo glass and I could have simply bolted the snorkle down once down. But as I said, I had to go with plan B.
Air box all painted and ready to go.
I laid some strips of glass across the base and glassed in part of them as shown above. I left loose ends that will be used to glass onto the foot.
Snorkle head
I put the foot back on the fender and you can see the loose ends I'll use later.
Once I mocked the snorkle back up on the Durango, I used the loose ends and glassed them over the foot. Then once cured, I removed the whole piece and trim the base around the foot.
Then started glassing the base and foot together.
Lightweight bondo is applied to start shaping and smoothing the foot and blending it in with the snorkle.
I duct tapped the fender again and applied bondo all around the bottom edge of the foot to create a good fit between the fender and the foot.
Bottom of the foot. You want to lightly sand this some to smooth it out and create a nice edge. But donot sand to much or you will lose your tight edge.
Snorkle finished and mocked up for a finally fit.
The flex hose was put in the fender from behind in the hole in the next picture. Then you can reach in and start pulling it to the hole and fit it has shown.
This is where it will connect to the air box.
Mocked up the air box and fitted the tube from the throttle body in place.
Mocked up the snorkle again to design a way to secure it at the roof top. 
Removed the snorkle and used th polyurethane to seal around the flex hose. If you keep your finger wet, you can mold the polyurethane all around the seal, pushing it down into the gap to create a great seal. 
I cut a small 1" x 2" square hole in the back of the snorkle where it bends up from the roof. I glasses in a piece of thick Starboard. I'll use this to drive in a screw to hold the top bracket. This gives it strength so the screw will not pull out through the thin glass. I also positioned the snorkle to where I could pour resin down the top of it and let it pool around the inside all around the starboard piece.
I did the same thing here after the resin I poured in the top cured. This is the foot. I place the snorkle on end and pour resin down the flex tube hole and let it pool all around. Once cured it gave the front of the foot a loot of pretection by making it solid. I only filled it to about an inch below the hole.
Plan B! I used 3 1/2" screws to attached the snorkle to the fender. Before I place the snorkle on, I used the polyurethane and ran thick beads all around the original hole for the bolts, and all around the fender where the foot will sit. I applied a lot of polyurethane to create not only a tight seal, but an air tight seal. I also ran beads of polyurethane on the foot itself all around the holes. I then used the wipes to clean up all the eccess polyurethane that squeezed out.
Here is an aluminum bracket I made to hold the top of the snorkle in place. I ran a bead of polyurethane under both parts of the bracket then screwed it in place and used the wipes to clean up the exccess. The stainless hose clamp was put in place to hold everything until the polyurethane cured for 24 hours.
Once cleaned up, I now have a great seal around the foot. See final pic below to see what I did about the ugly screw heads.
Close up of the seal.
I let the polyurethane cure for a few weeks since I used so much under the foot. I was then able to remove the ugly screws. Once removed, I drilled out the holes to fit these plastic rivets in place. As I said, they polyurethane is not only tough, but once cured, it is a super adhesive and the snorkle will not come off.
The plastic rivets are X-Metal Rivet Black from the company that makes grill for trucks. These are the rivets that go around the grill. You can buy them for 99 cents each from the company. The part number is 6710000-1B. Lot# XMT RIVETS, just Googke then to find the company if you want to use them.
You can basically put in type of bolt in the three holes. Just use the polyurethane to hold them in place.