You might be able to have a pair of wheels, one at the top and one at the bottom -- maybe with a flange on them like a railroad wheel. Have the spacing such that the film sprocket area rides on the wheels and forms the cylindrical shape. If they are low friction and all the other parts of the film path are lined up, you might be able to hold a cylindrical shape that way. The wheels need to be on bearings without a shaft between them so there's nothing blocking the active part of the optical path. That might tend to call for some machine shop type work to fabricate parts. For a one-off project in 35mm size film, those mechanics are going to be small and tedious to work with though.