Why would you need the film plane to be curved, when you are basically using a fish-eye lens. I would think the curvature might cause part of the negative to be out of focus, and have reflections if the radius is too small (ala your example w/film can). I think a larger radius so the film isn't acting like a mirror (say, oh, 2" or more)for the panorama effect. For making film guides easier,I would run the film on the outside of the curve and past a cut out "window" the size of your choosing. If I didn't have to be somewhere shortly, I would make an example for you.