Not a fan of the foam core idea. If it's featherweight, be hard to keep steady outdoors.
Had this beauty idea some years ago to make a bunch of 5X7 cams out of those styrofoam coolers hospitals use to ship medicine in. Made a bunch of film holders, made maybe 30 pinholes and picked the best ones for hole to film distance, did all the math.
Problem was, the containers were white. Nothing a few cans of matte black paint won't fix.
Spread out newspaper in the garage and gave them the first coat. Came back half hour later, and they were melted. Turns out toluene and styrofoam do not mix well.
I don't know if foam core has the same issue with paint or glue, but might want to test first.