I've made several 4x5 pinhole cameras from foamcore and a glue gun using dimensions given here: http://pinhole.stanford.edu/foamcore.html. I used flat black spray paint inside the camera and covered all the joints with black electrical tape to prevent light leaks.

I tried using weatherstripping to make a light tight seal against the film holder but it didn't work for me, I still got light leaks. Now I just use black electrical tape to seal the edges where the film holder meets the camera. It works great and I can reuse the tape 6-8 times. Then again I'm doing IR pinhole with 20 minute exposures so a few extra seconds to seal up the film holder is no biggie.

To mount the camera to a tripod, I sunk a T-nut into a piece of scrap wood and attach the camera to the wood with big rubber bands that sometimes come wrapped around the mail. Changing the orientation of the camera is easy, portrait or landscape or even laying the tripod on its side for a low shot.