Simple adjustable z-endstop for Vellemann K8200

This is an improved version of http://www.thingiverse.com/thing:400280:

- The disk got a knurling for a better grip
- The box now features two nut pockets to stabilize the long screw

You will still need the switch holder from http://www.thingiverse.com/download:682175
*Update 2023-01-06:* Seems like this isn't available anymore =(
If anyone still has those files, please comment.