Figuring out the space available for button PCBs and wiring to the main board requires accurate measurements of the partially assembled LEGO kit. Images of half-finished builds are helpful for rough measurements, but really we need a full 3D model of the build.

Following a build video that a LEGO reviewer posted, I was able to virtually build the set in the excellent BrickLink Studio software. This then let me export the full 3D model as a .obj file, suitable for inspecting virtually and 3D printing physical size guides.

Inspecting the model makes it clear that the space available underneath the buttons is barely larger than the buttons themselves. And with the D-pad, there is a rubber gromit that needs to fit through the middle that allows the part to pivot correctly. 

With all this taken into account, the resulting boards look like this, with a very small margin for error.

Printing out the PCB designs at 1:1 scale alongside 3D models of the parts they needed to fit into helped give more confidence that everything should fit.