Use my design and get a proper CNC machined case. [...]
I would suggest that you adapt the design so that it is milled out of a
single piece of wood per hand. You can make it a single piece if you omit having a plate.
A plate is structurally unnecessary because the PCB supports PCB-mounted switches, and is more a question of personal taste. You could also make plates from laser-cut 1.5mm acrylic. The plates don't need to be an integral part of the case. (the switches were made for plates 1.5 mm thick, not 4 mm ...)
The bottom of each case would be a single sheet of plywood, plastic or sheet metal. Push/glue metal grommets into holes milled into the bottom of the top case and close with bolts.
Beware that in the current designs posted, from Litster and czarek (who adapted Litster's design), there are errors in the stabiliser mount for the innermost thumb switch. That is something that I think that you should fix before starting manufacturing.
There are also several versions of the PCB, with or without support for PCB-mounted stabilisers. If you don't use PCBWing for manufacture, I suggest that you remove the text "Manufactured by PCBWing" from the PCB file...
There are bugs for the outermost columns - they were supposed to support 1x1 keys, but don't because some soldering holes were placed wrong in the design.
Know also that the keyboard was designed for a smaller case in mind, for which there are screw-holes in the PCB for - and those are unused in Litster's design. If you want to, you could use them.