Regarding the inside of the camera, I'm fond of using black adhesive craft felt, found at craft stores in the US like Michael's or Hobby Lobby.
For making the camera back, be sure to include a shallow groove, along the side of the opening where the film holder goes, that mates up with the raised ridge along that side of the film holder; this provides the crucial light seal along that side of the holder. The other three sides of the film holder/film back mating surface should be protected by an overhanging ledge from the camera body, preventing direct light from hitting the mating surface. I like to cover the camera back mating surface with adhesive black craft foam, again available at craft stores -- with the caviate that it works well for pinhole lenses, less well for refractive lenses that require a very accurate focusing distance be maintained between lens and film holder; the craft foam is a bit squishy.
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.
Thank you for another great ideas - particularly your tripod mount, glarsson, sounds like a great thing - to make a tripod base to use with several pinhole cameras might really do the trick!