This is a base for cut-out card minis, or standees as they used to be known.

The slot has a gentle S-bend to hold the base of the standee by tension.

I printed them at quite a low resolution (0.24mm layer height), not only because they print quite fast that way, but because the ridges of the individual layers also help to grip the bottom of the card figure.

They're 20mm in diameter, and intended to be glued to 22mm fender washers for a bit of weight and stability. They'd still work without that, but without the extra weight the standees will tend to fall over a lot in the slightest breeze.

Edit 2023-04-03: I've added 30mm and 40mm versions as well.