I agree about designing around a sliding box with short bellows for focusing.
A while ago I sketched up a telescopung box with bellows scrounged from a junk 8x10 camera.
In This case the bellows were to be out front, focusing the lens (smaller, lighter, easier to make).
The only downside is you need looooong arms to reach the focus knobs...
After I created a camera obscura in the wife's sewing room, I set this idea aside.
Here's my original sketch...