Hi everyone.
I don't know if it's usefull or not, but...
I've got some Testor and MM acrylics. The caps almost drove me insane! The paper insert sticks to the bottle, etc...
Tried some bottle caps from soda bottles - works great! Especially Coka-cola 0.5l bottles where the cup is a bit shorter then the regular one. First time you try to screw it on it will seems croocked, but plastic is weak!!!
I filled one up with water (which is thinner then any paint), put a Coke cap, fliped the bottle and left for a few days, not a drop got out.
These are plastic, and you can always replace it.