This is my slightly overengineered door knob sign. Helps me and my kids signalize to not disturb. ;D

It works only with d=20 mm door handles and holds pretty well.

You will need 4 pc. 10x2 mm magnets some superglue and ideally 3 colors of your choice.

Just paint the front and backside in your slicer with your desired colors.

Prints completely support free! Watch out to match the polarity of each magnet with each other!