Buy sheets of vynl, Goop glue. Fold the vynl over, glue a 1/2"
flap down, to itself, allow to dry, then you glue the under/glued flap part to the face of the vynl to make a real good glued edge .
Get an inner tube from a car or bike, cut out the fill valve,making a large circle of rubber around the valve, Goop glue this onto your vynl bag for the air fill valve.
Who knows? This might all work?
A vynl bag filled with old carpet padding might work too.