Mini magnetic clutch.

Mini magnetic clutch for gearbox:

https://www.thingiverse.com/thing:5445942

For the assembly you need 2 625zz bearings (used printer rubber wheels) and neodymium magnets 3mm in diameter and 1.4mm long, the force of the clutch depends on the number of magnets to be used or you can change the magnetic axis for a larger one with more cavities for magnets.

https://youtu.be/90cXZODVh1Y