I have tried the method Paul describes and did get a decent seal. Filling the bag with water shows it to be watertight, so far. Do not know about long-term durability.
Juuuuust wondering, here - If anyone has access (dads of teenaged daughters, do not get caught here) to one of those 'hair-straighteners' I see advertised. They appear to have heat from both sides, are called ceramic straighteners, and would possibly seal from both sides, being more effective??

Is anyone willing to do a trial - secretively, of course!! I would, but my daughter has straight hair, and does not (yet??!) own a curling iron.

kelly t

: Get a sample and see if you can seal the seams with heat instead of glue.
: Cover the material with aluminum foil or baking parchment paper ove the
: seam area and go over it iwth a hot home clothes iron. the aluminum foil
: or parchment keep the melted pvc from sticking to the base of the iron, so
: it can slide neatly. Play with the temperature setting on the iron to get
: a good fusing on the seams.

: PGJ