I think that would work. At 6.5x9, it may also actually be possible to use a sheet of film directly without a septum, letting the springy bits that apply pressure to the plate hold the film in place. The film might be rigid enough to support itself in a flat condition.
The glass doesn't actually need to be of precise thickness; those springs and the front lip of the holder should hold its front surface in the right place. However, you'll need to figure out how to make sure the film stays in place on the surface of the glass.