But not in the light! Back in the day, I did a number of these things and the paper backing isn't enough to guarantee light tightness. You need to open the cassette in the dark. The film itself should fit on a 35mm reel. The frames are square, and the sprocket holes aren't really sprocket holes. Like Alex said, the holes are there to tell the camera when the film has advanced the proper distance by engaging a pin in the camera.