Lot's going on, but I have been working on this project.
Originally I was going to use a standard back plate for the control cavity, but noooo. That would be too easy, said my ADHD (note: ADHD has been known to lie. Repeatedly). That being said, I started (over) thinking about the possibilities.
On this build there are three places where an allen wrench is needed for adjustment; 1-Truss rod, 2- the string locks on the headpiece, and 3- saddle height adjustment. I thought it would be cool to include the wrenches with this build. Even better if they were mounted onboard, in this case inside the control cavity.
I have built instruments that use rare earth magnets to attach various covers, but I discovered that they sometimes don't work as well as planned. Due to this, I no longer use them solely for attachment purposes. I decided to design the rear cover to rotate away from the rear control access. I could also make the cover large enough to allow for a second compartment to house the three allen wrenches.
After drawing out what I felt would work, I made a template of the cover. I then drew out the location of the control and wrench cavities. I also added a pivot point at the lower treble side bout.

I was also trying out different positions for some rare earth magnets to help hold the cover in position.
Next, I made a template to allow me to route out the area for the cover (I haven't cleaned it up with a spindle sander at this point).
I routed the wrench cavity first, taking it down past the thickness of the cover plate.
I routed the back for the coverplate on my table router. In hindsight I should have used a plunge router, I ended up routing too deep, I had some tear out as well.
Next is building up the cavity to the proper height, the making the cavity cover.