Summary

I made this mold to cast ceramic signet rings, it should be sand-able to fit fingers from 14mm to 20mm, but you'll have to shape it to the proper size after casting. 3 piece mold, I can't model the alignment pegs well so expect to sand a bit to get them to fit. Technically, you should be able to just print one side, twice; they are interchangeable, but I modeled them individually.

I made it to produce a "Thicc" ring or a "One size can be made to fit most" ring that can be sanded down to be the right size. Sand the inside of the ring band to slightly larger than you need (it will shrink when it dries, more if it's fired), then sand the outside down to somewhere between bulky and too thin. I suppose you could print the dummy ring, shape it and do lost PLA casting as well.

This kinda sucks for clay, dries too slowly