Z-extension for Maker Select v2

Chopped off the lower part of a [recent Z rear brace with extension part](https://www.thingiverse.com/thing:2990990) I did, in case someone wants the Z extension without the Z rear brace.

This puts no stress on the upper cross-brace of the frame.

**TIPS**

- I printed these with large flat side on the bed (the side with the hex nut holes). It puts a lot of supports in the 2mm gap -- you can remove these with thin needlenose pliers, but biggest help was to use the scraper that came with the printer to clear the supports from the gap.

- **WARNING!** There is very little clearance between the inside edge of these and the Z frame holder! So when hitting those high Z heights (> 160mm) be sure that the lower screws on these parts do not stick out. Recommend M3 x 10mm screws. I only had M3 x 12mm so I added another nut on the 'head' side of the screws to keep it from sticking out the other side.

- The X axis brace is still mounted on the front using the top holes. I think it's b