For #FreeCADFriday (UTC), I have some silly little potentiometer shaft fridge magnets. They let me try out different control layouts to see if there's room to turn a pot without bumping another one, and what controls a patch cable will obstruct.
This kit has already gotten very heavy use as I plan out the panels for my first group of DIY #eurorack modules. I expect I'll keep growing my own set of these, to make design checkout super easy.
https://www.printables.com/model/1528701-fridge-magnet-potentiometer-shafts