I went to seattle fabrics and looked at some materials. I believe I looked at some 200d urethane coated nylon and wondered if it would hold air. If thats what flyweight uses, and I've never heard anyone complain about the material leaking, then it must work. I take it the urethane coating makes fabric 100% waterproof and air tight? On zpacks.com I found a material that is nylon coated cuben fiber. It's 2.92 oz per sq yd and is apparently very tough. It's waterproof, but I don't know if it would hold air under pressure. I sent an email to get some more info on it, but I know he isn't an expert on air bladders. I think I may have to buy some of the 200d nylon and make a small air bladder and see if it would work. Now the tricky part is figuring out how to seal the seams..