I forgot to mention; you will probably also need to cut a slot or some sort of relief to catch the ridge on the holder face along the darkslide side. This serves two functions. It creates a baffle where you don't have a light baffling corner around the side of the holder where the slide operates, and it catches the holder so that it doesn't slide out of the camera when you pull the darkslide.

Good luck. Pinholes are a lot of fun.