Lack side panel brackets

Building a Lack enclosure, sealing the sides with plywood & insulating foam, needed a way to secure them in place, was going to design a web corner bracket but found apalexander's design which was almost exactly what I needed.

- Removed the 1/4 spider from the bracket.
- Reduced overall size by 30%
- Re-positioned some mounting holes
- Reduced height from 20mm to 12.5mm

As the webbing isn't symmetrical, created 2 STLs, designated as "Upper Left & Bottom Right" and "Upper Right & Bottom