The PCB for the second version of this keyboard arrived:
This time I'm using switch sockets, so I decided to add an extra row and extra column to each side, to have more room with experimenting with different arrangements of keys. I also put the connector in the middle a little bit closer to the edge, so I can solder pin headers in there, and have a sort of a "clip" made of a double-row header, to connect the halves together, and still be able to disconnect them for transport. Oh, and since the sockets add a few millimetres to the bottom anyways, I moved the diodes to the bottom too.
Right now I ran out of the sockets, so the PCB is waiting for more to arrive.