I made this sometime ago, and someone asked on my video of them if I could share the design so I figured I'd put it here. I suspect there are better designed versions of similar things on the site already, but hey, here is mine

since the original design I got rid of the tab at the back, relying on the side top/bottom bumps for stacking alignment. Not included here is the french cleat piece, I just screwed a block with the necessary angle for my system on the backs of the ones I wanted to hang.

These need to be glued together.

the video of me making them: https://www.youtube.com/watch?v=H-eHP_xwkfA