I got things more-or-less put together tonight. I need longer axles; 20 cm was about 8 cm too short. But anyways, I now know how everything fits, and how the top will have to interact with the furnishings. Designing that will be simple.
The top will only have bridges pressing on it. The Keybox will not touch it. The top will have a perforation for the wheel, and another one under the keybox that will be accessible for when the axle or wheel need servicing or replacement. There will be minimal bracing– just enough to support the bridges, and something to prevent dry cracking in wintertime (something I have found is needed here in Minnesota — I’ve lost some mighty fine guitars to the dessicated air from December to February).
The top will vibrate freely. I am happy with how it looks in my mind’s eye.