Focal length would be the size of the box. But if this is a true pinhole camera you needn't worry about that. If the camera is big enough for you to get inside your options for reloading are broader, but either way you could consider a large light tight tent or bag that you could pull around the whole box during the swap out. I'm thinking of something like a magician uses, pulls up from the bottom. Mural paper comes in a square tube box that you can use for transport. Good luck!