I've posted twice about experiments with expanding polyurethane foam (here and here). Since I had a large can of the stuff I tried a couple of new experiments: I filled a balloon with foam to see if it could be used as a mold and I filled a bagel bag to see how well (or not) the foam sticks to the plastic.
This was not successful at all. I filled it simple party balloon with foam and sealed the neck with a cable tie.
I filled the inside of a moistened bagel bag with foam and left it to set over night.