Instead of a rubber band, what about the end of a balloon for a seal? I figure you could cut the end of one of those long balloons they use to make animals out of. So instead of trying to glue it to the inside of plunger, you could wrap the balloon on the outside, then use the shell as a clamp when screwing it back in.
I know, around here, ideas are better in pictures. Which makes sense, because it probably looks like I'm not making any.
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