Vinyl / PVC float bags are easy to make, and easy to seal. The key is using all vinyl....
Vinyl plastic for the bags, vinyl cement, and vinyl flanges / vinyl inflation tubes.

Potential failure comes when you substitute non vinyl components into the mix, especially the glues used.
Another potential problem is trying to cut corners by attaching the tube directly to the skin vs using a
proper vinyl flange.

I prefer cloth supported vinyl ( 18oz kayak skin material) to unsupported vinyl as it's more durable.
However, non supported vinyl, such as clear, is a bit easier to seal. Follow the instructions below,
and you should have no problems.

http://www.yostwerks.com/FloatbagA.html - PVC / Vinyl float bag instructions

Heat sealable nylon is a not as easy as it seems. I've had mixed success when making sponsons and
float bags. Heat sealing is a somewhat tedious process, especially on a long sponsons, and sealing
the vinyl valve / inflation tubes to the nylon sponson material is problematic due to the
incompatibility of materials used. It can be done however.

The biggest drawback to nylon heat seal float bags is the difficulty in reparing them vs easy to
repair vinyl. The advantage of heat sealable nylon is light weight and flexibilty. Though not so
critical in a float bag, it's more an issue with full length sponsons in a folding kayak.

Probably the best overall solution to float bags is to buy ones commercially produced as the cost
isn't a big factor. ... Unless you simply enjoy doing it yourself.

Below is a small homebuilt heat sealable nylon sponson.