The CNC router's firmware? The CNC controller itself is dumb as hell. It's connected via serial to an old offline computer that's only job is to pipe G-code over RS-232. Not even to "load a program" to hold in memory on the controller. Literally to stream the commands to the machine.
If the CNC machine is not connected to a network, and (I don't know much about CNC, but I do wonder) if one can provide a generic program, then just fill in the parameters before execution, right at the machine, the secret would be confined to just the CNC machine.
Regardless, provided the CNC machine is not networked, and provided it doesn't have any solid state (only RAM), one could perhaps load a large job to overrite the previous one.
Although, perhaps the CNC machine could be used to just provide templates, with small indentations for where the holes could be punched, and additional engraving to provide a link to a document on how to use the card in case of disaster recovery.
Sometimes CNC machines are networked, sometimes not, but the idea behind saving keys like this is that you only generate the appropriate bits on disposable computers, e.g. a Raspberry Pi with an SD card you can then destroy.
It depends on how much money you want to secure, of course, so YMMV.