2024 Update 2: I started from scratch to create a better model. Simpler, smoother, etc. Also a new print orientation. The previous version was prone to snapping in half. The new orientation (like a U) has the layers along the 2 planes with the most stress. A little sanding after printing helps with comfort. Three sizes, I, II, III are 1mm, 1.25mm, and 1.5mm thick.

2024 Update: Still using them. I've updated the S and M versions with a wider brim because the original would fall out. I can't tell you the last time I lost one.

Update: I am still using this design, reprinting a new one every 6 months when I lose or break it. I have lost weight since posting this design, so I have added Medium and Large sizes. I've also put them at the angle I print them (22.5 degrees from vertical).

This is a shim I designed to resize my wedding band. It is sized for my 6.5mm x 21.5mm id wedding band, but you may be able to scale it to fit your ring.

At my wife's suggestion, I printed it standing up, like a half moon, with support. This provides strength to the rims so they don't break off.