Another simple camera idea is to go to Hobby Lobby and find those photo/video storage boxes. They have one that's covered in black felt, including the lid where it wraps under and mates with the body of the box that also is covered with felt. Meaning that the storage box comes from the store already light-tight, all you have to do is cut a small hole in the center of the lid for the pinhole and shutter (I use a rotating piece of Masonite secured by a small machine screw and nut as a shutter).
The lid can be kept secure by several large rubber bands (or several small bungee cords).
The inside of the box is not quite tall enough for 8x10 paper, so you simply curve the paper slightly and it will fit fine in the bottom of the box.
This is the easiest box to convert to pinhole that I've yet found in a store.
Another idea are those black with aluminum trim file storage boxes at office supply stores. These have a slot and groove fastener for the lid, which you need to paint flat black, but otherwise these boxes can be light-tight as-is, provided the hinges are not flimsy and the lid is able to lock securely to the body of the box.
The ones I've seen at Staples are lined on the inside with a black, felt-like fabric, all you would need to do is cut a hole in one side for the pinhole and shutter, and fix a simple plate on the other side to secure the paper or film.
Or affix arm sleeves to the box and you could change out the paper in the field?
And I think some of these boxes are legal-sized, meaning they might fit 11x14 paper.