I am thinking of building a pinhole camera, probably out of cardboard this first time around. I'd like to make it 4x5. I got the specs, focal length, pinhole size, etc. However, I'm not quite sure how to get the film holder light-tight to the "body." Any suggestions? I'm not that familiar with 4x5 film holders so I don't know how much built-in light-tight-ness there already is (ie - if I just get it up nice and snug on the "box," will that be sufficient?). Should I put some kind of felt on there or something?