Summary

Foot for t-slot. Only tested on 20mm, and many versions have different internal hole diameters, so you may need to play with it. I find you need to go .6mm or so bigger than the measured diameter of your hole for it to insert and stay. The alignment block keeps it from rotating and at some point I'll make that more customizable, too. Best printed in TPU.

Open the .scad file in OpenSCAD and go to town.