As a home brewer with a keg and draft system, I always found it annoying that I needed to remember to purchase bottle caps if I wanted to share my homebrew by the bottle. This cap system is an answer to that to produce an air-tight seal on any pop-top style bottle.
The cap needs two non-printed parts: a nickel (preferably a clean one) and a food safe garden hose style gasket. Why a nickel? 3D printed plastic is rarely air tight. Nickels are cheap and always the same size, so I figured why not have the gasket press up against a nickel instead of plastic? The nickel creates that air-tight seal when pressed against the gasket/bottle top.
Print without supports with a part cooling fan running at 30%. The nickel and gasket are press fit into the plastic.
If you would like to make any changes to the design and can work with Autodesk Fusion 360, it is available at https://a360.co/2yl5ltb
I printed in PETG, but any plastic should work. In order to prevent the threads from warping, I would recommend the part cooling fan run at a constant 30%.