It is certainly desirable to paint the interior black to prevent unwanted reflections. Also, some cardboard and thin plywood are not completely opaque. There are many ways of eliminating light leaks around the filmholders. One way is to design your camera like the backs of other large format cameras, using their light trap methods. T-nuts, available from most hardware stores, can simplify making tripod sockets. Tripod sockets on two sides of your camera permit it to be used in both portrait and landscape orientation. Composing is often done with lines drawn on the sides and top of the camera that show the angle of coverage. More elaborate viewfinders can be improvised.
Your questions may have already been answered in more depth on sites like http://www.f295.org/Pinholeforum/forum/Blah.pl? or http://home.online.no/~gjon/pinhole.htm.