I saw a guy turn an old box camera into a 35mm camera. What he ended up with was exposures the entire width of the film and about 3 inches long. Funky results. Of course he had to cover the red hole in the back. So if you do 35mm pinhole work with such a camera, you will also have to cover the hole. I have used 35mm in a Holga with a 120 mask in it, but Holga makes a solid back, one without the flippable red hole.