another spool holder for i3

I wanted to create an own support for my Prusa i3, so this is the result. FEM tested with 100N (~10kg), that should do.
Fits on 6.2mm wood frame / 8mm axis.

*Update: added a version with anti-warp features on the outer edges

*Update: added versions for 10mm and 3mm frames. Although these are not FEM tested, they should be able to handle a spool :-)