The pop button worked worse than the last one. The TPU is not flexible enough at the minimum thickness I can print.

Cura currently turns walls thinner than 0.41 into thin air for me.

The one on the left is the previous attempt with square corners. That was a mistake as when it's inverted the corners are a shorter path so it distorts.

The one on the right has rounded corners, but won't stay inverted as the TPU is not flexible enough at 0.4mm thickness.

If this is going to be spill resistant, it has to have a one part design for the top or be glued together which adds a whole other set of problems.

I think I may experiment with making molds and using real silicone.