The computer contains 18 vertical boards, all joined by a horizontal backplane: see the log Structural sketches
The vertical "bitslices" boards can be easily made as 20 identical boards so they are quite cost effective. However the backplane is large, complex and costly. Prototyping will be hard...
Fortunately I found 18×30cm perf boards on ebay: cheap, large and easy to fix (though wiring it all will be another marathon).
Now, there is a little issue : how can one wire everything on both sides of the board ?
The point of this log is to document the solution : it's not one board but actually two similar boards where the wiring is on one side and the connectors are on the other side. Both boards are then joined together, on their wiring side, using mating connectors, so neighbouring cards can communicate.
(insert sketch here in the future)
Overall, I get a 300mm wide backplane, which is enough for the width of 9 bitslices: one inch or 25.4mm is enough for one board, 9 boards take 23cm and there is margin on both sides. For long bitslice boards (the DRAM will take some room...) I can add more such cards. All the wiring can be inspected and modified as needed.
This system can be adapted to other implementations of the YGREC architecture :-)