speed washer

This is a "speed washer", most commonly used on the outside surface of skateboard bearings (one on each side). The washer provides solid contact with the bearing inner race, keeping the covers from rubbing on a truck or nut -- improving speed.

They're very thin and small, to minimally impact spacing.

These also work really well for fidget spinners -- if your favorite caps don't already have a small spacer built-in. Some do, some don't.

This is a tiny part; bed adhesion will be important. I use PLA on glass, with some glue stick (PVA).