What I would do:
Make a Eurocard PCB with the DIN41612 connector at the end (lets call it adapter board). Design the board, so it can take 2 Orange PIs, 5ish raspberry Pi zeros, 4 CHIP. or your choice of computing. Orange PI is mounted upside down on the adapter board, RPi zero has the connector soldered on the bottom, and USB pins tapped with a pogo pin. Route 24V, maybe 12V to the adapter board, have an onboard POL converter. Design a backplane, it routes power + ethernet to each board. Each adapter board has an ethernet switch, in case of necessary. Maybe it even has hot swap control. The backplane has 21 connectors (AFAIK, that is standard). I reckon, you can fit 42 RPis, 105 RPi zeros into a 3U rack. All mechanical parts are off the shelf. Cooling vertically. Hot plugging the cards, if you feel like. Mix different cards, if you feel like.