Perhaps a large enough changing tent could enable the camera to be reloaded in the field. Depends on the cost of film holders (used) versus a changing tent.

I built a portable processing box (with arm sleeves) for processing 4x5 paper negatives, something like this could perhaps be adapted for reloading smaller cameras in the field without the expense of film holders, especially given the inexpensiveness of materials when building it by hand.

I had an idea once for a box camera with storage compartment behind the film plane, accessed by the side of the camera being removable via a lid, with the camera inside a changing tent, where the film at the film plane would be swapped out by one from the rear film compartment, by hand, inside the tent, then the lid reattached. The problem is the changing tent needs to be big enough to permit the film to slide out, without being touched and/or scratched. There's always things about these DIY ideas that remind me why film holders were such a great invention.

I have successfully built three pinhole cameras that use a falling plate mechanism, where a stack of film plates is at the back of the camera; after the front one is exposed, a lever or knob is slid, and the camera tipped forward, making the front plate fall face-down into the bottom of the box. The two problems with this design are: 1) the focal length has to be longer than the height of the film plate, ruling out wide angle designs; 2) if the box camera is excessively jiggled or tipped upside down the whole mechanism is jammed and quits working.

~Joe