Tupperware Eco 500ml 16oz cap for bottle

My daughter got some tupperware ECO bottles for school, but these were only with the drinking cap, which is not usable for water with gas as the pressure opens the cap and the liquid will flow out of the bottle.
So I decided to design a cap for the tupperware ECO. I made it for the german version (500ml), but on the bottle there is a stamp which says 16oz. Because of that I am of the opinion, that you can use the cap in the US too.
Don't use PLA! PLA is not flexible enough and also will get s