This is my attempt of a bottlecap. its not 100% waterproof if you dont add a rubber layer. I just quickly tested with a sheet of highgrade paper and it was pretty waterproof. The PLA is just too hard. It also worked on my glass-bottle.

I made this to test the seal as i am working on a bigger project. I added the CAD .prt file as i had an extremely hard time finding one.

Let me see what you can come up with.