Z extension of Z brace

Wanted to do the SWD2 Z back brace mod but also wanted to do a Z extension mod.

So after a few days I made one in Tinkercad. Yesterday come to find out [kehlebract](https://www.thingiverse.com/thing:2588219) already made the same thing (didn't think to look under remixes).

Anyway, here's my version of the merge of [ShadowWeaver2067's Z-axis rear mount](https://www.thingiverse.com/thing:2391089) and [Azza's Z-extensions](https://www.thingiverse.com/thing:1444979).

My first thing on t