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.