another simple pinhole camera design is made by 2 boxes that fit inside eachother ( like sheet film boxes ) ... one has a hole in the front ( for the pinhole ) the other has a handle on it to aid in pulling it out, or pushing it in. i have a 8x10 pinhole like that, and it works great. you can also use a shoe-box.

it doesn't have to be made of wood, it can be made of black cardboard/foamcore/matboard or pretty much anything ...

you can multilpy whatever focal length you want to use by "3" to find out the image circle ... when i was making a 12x20 pinhole that was the number given to me so i would avoid "bowing" my film ...

good luck!

-john