Case design improvements - bottom plate

Two things I think would finish off the bottom plate a little better:

1. Thickening the bottom plate a bit (and/or just beefing up an area near the center of mass) so it can accept a [1/4"-20 8mm deep threaded heat set insert](https://amzn.to/3WQCDu4).
2. Along with the thickening, recess the two screw holes for holding the bottom plate to the upper case, that way the bottom can sit flat on a surface with no wobble.