I built this pinhole camera several years ago. The black box is made of matboard and it fits into a groove into the wooden frame. A standard 4×5 film holder drops into a velvet-lined slot for the exposure. There’s a tripod screw in the bottom of the frame (the rock shown in the picture is just to prop up the front for the photo!). I didn’t glue the box on the front, but just squeezed it into the groove using matboard shims to pressure-fit it, but with age and use it has gotten loose, hence the rubber bands. I should probably just glue it.
The focal length is about 4 inches, and the pinhole is about 0.017″, for an aperture of about f/256. The field of view is about 75 degrees, similar to a 28mm lens on 35mm. It is predictable enough that I have shot Ektachrome transparencies with quite good exposure.
Viva la pinhole!