Once the SBC worked, a subrack was needed, containing the power supply and the backplane. I got a cheap parallel backplane, modified it as needed, and got a used subrack from Ebay, modifying that as well.
A common 3 U subrack, currently holding the SBC and the power supply.
The backplane has 10 slots, bought from Reichelt for 20 EUR.
I soldered faston connectors to it, which appear to be popular backplane power connectors.
A cheap frameless power supply mounted to a lab card with a simple front plate.
A DIN 41612 H 15 power connector with the standard pinout for wall power and +5V allows to exchange the power supply easily. Compare that to "modern" PCs!
I had to modify the backplane for the interrupt enable daisy chain by disconnecting all traces for C11 and C16 and then connecting C16 of each slot with C11 of its right neighbour (viewed from the front side). The SBC must be plugged into the leftmost slot of those using the interrupt chain. If there is a free slot in the chain, the two pins on it must be bridged.