for the 4x5 you could check out how they do backs on the leonardo at the pinhole resource. it is pretty simply done - a cut-out in the back of the camera/box and lined with light-block and 2 sliding blocks to secure the film holder. i think the cameras made by the lensless camera company ( santa barbara camera company ?) are done the same way.

http://www.pinholeresource.com/agora...roduct=Cameras

good luck!