Two options for attaching back (cleat) on the two piece version
CA glue. Designed with a mating registration surfaces to locate horizontally in the center and to give lots of strength for almost all purposes with just CA glue
Hardware, made for ¼ bolts
2 x ¾" long ¼ 20 bolt
2 x ¼ 20 nuts
Design details:
Holes on the bottom serve to prevent dust accumulation
Made for 3/4" plywood (note: 3/4" plywood is usually closer to 0.7" thick, or around 18mm).
Designed around standard imperial specs common in most US shops (sorry world)
Could easily be remixed for just about any mounting system
Print details
Use snug supports just for the vertical holes, no other support needed (might even be able to get away with eliminating those)
Love the design of the 4x4 and almost had a perfect print, but the supports failed on me. Any chance you can make the 4x4 with a separate cleat option like the 4x2?