Thanks to all who helped.
I'm still mulling over the design. I guess I'll make more than one, anyway, so I can experiment. I feel drawn to a design which uses curved film plane - not too difficult to implement, and could yield interesting "panoramic" results.
I'm currently considering a more practical way of attaching the back: some DIY designs I saw use as much as 4 screws to attach the back to the camera: not very practical, I'm afraid.
But, I decided to "test the waters" first with a simpler design: a square box that will use my existing roll back & the Grafmatic I have, which will do just fine for starters, I guess