It seems the idea of a simple shutter got lost along the way, though I did appreciate the ingenuity.

As for the matter of being committed to horizontal exposures as mentioned on page 1, simply calibrate for a vertical exposure, and when you rotate the back subtract 1/4 for a 4:5 proportion, or 1/3 for a 2:3. If you have to rotate the camera, same calculations, just mount the shutter to allow rotating it 90 degrees.