This is a lens cap that will fit canon EOS-M / EF-M lenses.

I searched everywhere and the only model of this I could find had floating geometry so instead I made my own!

The fit is really snug and holds its place on the lens, you may need to fiddle with your slicer settings to get just the right fit. If you're using Cura an easy trick is to change the "Slicing Tolerance" setting under "Experimental Settings" (you may need to enable this section).