The key caps have arrived, so I finally can do the measurements for the placement of stabilizers, figure out where the USB socket and the MCU should go, and maybe finalize the PCB. Turns out the 1.5u keys don't have stabilizers, so that leaves me with just the 5.5u space, 2.5u shift, and 2u enter and backspace keys. I can also test spacing between the keys, to make sure the caps don't collide.
(Sorry for the hair, it's the shedding season.)
I was actually quite surprised that there are many alternate key sizes for some of the keys — specifically the enter, shift, alt and fn keys. That gives me a little bit more flexibility.