So after a few iterations I have a simple PCB mounting bracket. I bolt the PCB to it with m3 threaded inserts, then the bracket itself bolts to the slot box with m3 inserts too. Of course this approach postdates the slot box print so I had to retrofit the bolt holes with a drill which was kind of a bummer but it worked. I had to use some flathead bolts so I could countersink them, and they need one size smaller hex wrench than the rest of the keyboard, which is unfortunate. But now that they're fastened there's not normally a need to unfasten them.

So I think the build is as done as it's going to get for now and I can move on to writing it up in a blog post. I'm also starting to learn the engram layout from scratch but it's slow going so I'm still adding home row keys.