A very basic "thumbscrew" to go on the head of a M3 machine screw. 10mm diameter x 5mm high.

Instructions

Basically you just print a bunch of these, put a flat edge on the heads of some M3 machine screws, and push the screw heads in nice and hard with a screwdriver (a hole in a block of wood helps).

I use these with nylon M3 screws, but if you used a grinder to put a flat edge on the head of a metal machine screw it should also work.