A few days ago I asked for advice on how to make a cap for my MXR M222 talkbox. I got a lot of good suggestions, including looking for a matching rubber stopper, 3D printing with 95A TPU for flexibility, or turning something on a lathe and embedding a silicone O-ring. But then Paul Ganssle saw my post and reached out: he saw one of these on Thingverse and offered to print me one.
He brought it by yesterday, and it's great!
It fits just right: not too hard to slide on, but doesn't feel like it's going to slide off by accident.