Monoprice Mini (MPSM) WK9.1 extruder base with access to set screw

**Update 23 Aug 2021:** Added a WK9.2 version which doesn't need the separate nut for the collet, and uses printed-in M6x1.0 threads. Based on my experience with printed threads it should be perfectly fine.

I've been using my Monoprice Mini pretty heavily, and started having problems with inconsistent prints that seemed to be related to extrusion problems. Investigating, I found out that the teeth on the extruder gear were worn out, so I ordered [a replacement](https://gigdigit.com/extruder-g