Random thoughts:
I seen 4x5 pinholes made from black foamcore that mate to a 4x5 film holder. Rubber bands hold the "body" to the film holder. I think best bet for cheap 4x5. Similar to Todd's 3D printed version though not as fancy. There are people/places that will 3d print from a plan you provide.
A friend who does excellent pinhole work uses a 6x9 Agfa Isolette with the lens removed and replaced by a pinhole. The shutter remains. Great way to repurpose an Agfa Isolette with frozen focus. I just did this with a 6x6 folder but have yet to test it out.
I just built a simple 4x5 pinhole from a metal cookie tin to do pinholes with my granddaughter. I used small magnets to hold paper in place. Would work for film too. I've toyed with the idea of using a large tin with 5 holes. One in the center for an 8x10 and one toward each corner for 4 4x5s (probably need a baffle inside). The idea is that you can do 4 test exposures with a single load/unload. Then your "final" exposure with the center hole on a fresh sheet.
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