<p style=”text-align: left;”>Thanks – I take your point about the difficulty in getting an airtight seal. I have made air bladders out of TPU film (rather than TPU coated nylon). The material is stretchy and kind of clings to itself, so it wasn’t difficult to create an airtight seal. Maybe I’ll try and bond a tube of this stuff to the TPU coating and see if it works. For a 50cm opening, the zip is probably the way to go, but this could be useful for a narrower opening just to provide access for leaks and repairs.</p>