I suggest X-Ray film.
It's on the slow side, and either blue-sensitive (like collodion and the earliest dry plates) or "green sensitive" (actually orthochromatic), and it's very inexpensive compared to even Fomapan.. It's slow, however (ISO 25 equivalent, give or take a stop) and slower if the light is yellow or red (golden hour? You'll need two for your exposure!). Given Fomapan's abysmal reciprocity departure, the green sensitive, double speed x-ray film may actually be faster in a pinhole camera.
Another good alternative is ordinary enlarging paper. This will need trimmed to fit in film holders, but if it's just going in a box, held in place by tape loops or similar, that won't matter -- and you can contact print to a positive just as you'd do with film (though you'll need a bit longer exposure). Still slower (ISO 6 to 12 for most versions), but no reciprocity departure for exposures measured in minutes. You can also buy Ilford/Harman Direct Positive, that makes a positive image when processed in normal print developing chemistry. It's expensive compared to normal enlarging paper, but there's something to be said for a Polaroid-like unique positive print.