If you want to make multi colored prints the slicer should automatically assemble the “ball” and “cracks” STLs.

I made them for 20x3mm magnets to work on thick windows. I also uploaded a version without magnet holes so you can cut in holes in your desired size (negative parts in slicer) or use both-sided adhesive tape.

Works also good as fridge magnet …

I originally posted this model at Printables.