You could make a larger opening and add a frame to put aperture cards in it.
Types like these: http://re-inventedphotoequip.com/Sit...gurations.html
Maybe Reinhold (the lens builder from this website) can make you a card with a smaller aperture.
Or just make one yourself from wood with a shim in it.
This way you can adjust your pinhole size easily if you want to make the focal distance smaller.
Projecting the whole image on the back wall or making a print with photo paper at 1 or 2 feet from the pinhole or ...
You could even add a red filter to position the photo paper before making the actual print.
Use Duct tape on the outside as a shutter ;-)