I couldn't get the 'spring' part to print cleanly and be springy enough so I changed tactics and hot glued a magnet with a hole in the middle underneath and used a small magnet with a handle as a stopper. Works great.
Update: hmm, the magnet I used on the bottom has a metal casing around it, and it's not rust proof. So after just a few days of leaving water in it (testing for leaks), it has started to rust slightly. Should be solvable using a magnet without the casing, rather than just whatever magnet I had handy. (edited)
I couldn't get the 'spring' part to print cleanly and be springy enough so I changed tactics and hot glued a magnet with a hole in the middle underneath and used a small magnet with a handle as a stopper. Works great.
Update: hmm, the magnet I used on the bottom has a metal casing around it, and it's not rust proof. So after just a few days of leaving water in it (testing for leaks), it has started to rust slightly. Should be solvable using a magnet without the casing, rather than just whatever magnet I had handy. (edited)
Works like a charm.
Printed with a 0.12mm resolution, 20% Gyroid infill and in PETG.