You should first decide what type of sensitive material you are going to put inside your camera. Main choices of camera/material are:

• Coffee can or shoe box loaded with photographic paper

That’s about the cheapest you can make. Exposure times can be long (one min or so) and you get high-contrast paper negatives that are easy to develop and contact-print. Main drawback is that the user can take only one picture before returning the camera to you.

• Some kind of box loaded with sheet film

This is optimal quality-wise, but more expensive, harder to develop (total darkness) and has the same drawback of only allowing one picture. Exposure times of ~ 1 s (sunny) to 15 s (overcast).

• Roll-film loaded camera

Easy to develop, many pictures per roll (from 6 on 6×12 to 36 on 24×36). Sharpness is OK on 120, but poor on 135. Main drawback is that these cameras are hard to build, because you need a means to advance the film.

I think I would go for the easiest solution and load some boxes with photo paper. Film would be better but you need more budget and/or time to build the cameras.

Edit: If despite the complexity you opt for the roll-film solution and you have access to a fab-lab, you may want to try the 6×9 pinhole camera I designed. The page is in French but you don’t have to read it: just look at the pictures and download the design files. You will need a laser cutter, a 3D printer, 3 mm MDF, black paint, glue, and obviously a pinhole. However, although it’s a very nice camera for personal use, making 50 of them would be very tedious!