I think the only issue you'll have is it takes up so much room. I have my 3" mounted so it goes along the top of the grill lid, and then comes down in the front. That way I can still sit things on the left side of the grill (just not on the front of the left side where the duct actually comes down to the grill level). Check it out:
I don't know if this will help you find what you need or not, but the duct that I used is a 3", and it's compressed. You won't find it with the dryer vents, but with the regular ducting. Home Depot and Lowe's usually have a section that is somewhere near the dryers and there are tons of different dryer vents there (some compressed, some flexible, some not flexible, all different sizes- 3", 4", 5", etc). That is not where you want to look. If you keep looking around, you'll find a section that has furnace and air conditioner ducting. There you will find a 3" ducting that is compressed to somewhere between 18" and 3' long. You can stretch it out (uncompress it, whatever) to around 8 feet long.
That is the stuff to use. Leave it compressed and it will be the perfect stiffness to stay in place. I had to use some scissors to cut a little off of mine though, but it was otherwise perfect.
I got this information off of one of these boards (probably this one) after I had used the wrong stuff on mine and had trouble making ribs. I was doing spares, and trying to do the 3-2-1 method. I was cooking 4 slabs, on some rib racks that I bought at Lowe's. Anyway, rather than wrap them all in foil, I just put them in foil pans and then covered the top in foil. And with that smokestack mod taking up the whole left side of the grill, I had a hard time fitting 2 pans on the cooking grate.
I went out and bought the correct ducting, redid the mod with that compressed duct so it is up in the top of the lid, and I am now able to use the whole cooking grate minus the left front corner where the ducting actually meets the grate.
