Generally, silicone has great elasticity and chemical and temperature resistance. I can only guess at the gauge, but if you can figure out a way to secure the opening, you might have a nice expandable, flexible and very stuffable waterproof pouch. I’m thinking along the lines of a mini boat bag where you roll the top and secure. Should be able to fit a camera, phone, and/ or wallet in there.

Silicone bakeware is all the rage, but I cannot imagine that this gauge could hold up.

Wonder if it would hold up to hot stone water boiling? Of course, you need it to remain upright. Maybe some grommets, some line and an overhead branch to suspend it off the ground.

Cutting your own custom gaskets, seals and o rings.

Tent or other gear repair, provides a nice custom sized patch –with the proper adhesive it should be waterproof and flexible.

Wonder how much water it could hold? With some silicone adhesive you could close the top except for a hole which would accept a tapered rubber stopper, like a cork. The smaller the hole the tighter the fit, presumably. You might end up with a pretty good water bladder. Don’t know about carrying it without some additional modifications or how well that rubber stopper would hold up to pressure if the bladder is compressed. Might be false economy to spend the time and money.

If it does turn out to hold a significant amount of water when stretched, with some more modifications (sealing, suspension water release) it could be a portable shower. I just don’t think the water inside the cap is going to stretch it enough to increase the capacity enough.

Cutting the cap laterally would provide you with some very large and tough “rubber” bands.