Try mat board, not foam core, for the film holder. Single weight mat board should suffice. Building up the film holder with slots for thin metal dark slides and slots for the film should result in a light and compact holder. Standard holders have a ridge that fits into a grove as a light trap. Draping a large black cloth over the back of the camera while manipulating the dark slide and exposing the film may eliminate the need for this. Taping a tab made from the same mat board as the holder to the film can serve as a handle for inserting the film as well as a light trap. A step on the tab makes an even better light trap. The tab and the adjacent surfaces of the film holder should be blackened to better block light. When loading film, you may first have to insert a piece of paper in the slot opposite the loading slot to guide the film into that slot. A knife, a steel rule, and a keen mind are most of what one needs for making pinhole cameras.