Think how many pinholes you could make out of $300 worth of beer cans.
Whoa! I have to admit my first reaction to that kit was "wow, a high-priced solution looking for a problem." But then again, it's the OP's money, and the beauty of hobbies is they don't have to be economically justified. :D You could mount a step-up ring of suitable size and use a second ring (or work with the old series adapters and retainers) to allow clamping in a pinhole plate and achieve relatively quick interchange for way less money. But, if the OP indeed has (or has ambitions toward) LF cameras, mayhaps it all makes sense.
To me the biggest problem is that the camera box to mount that turret thingy on should be mahogany or cherry with polished brass fittings, and maybe some decorative inlays, to be consistently fashionable. :whistling:
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.
I suspect there might be a similar problem with foamcore. You could use black foamcore to eliminate the paint, and the water-based glues - Elmer's, Titebond, etc. to minimize solvent exposure. Brushing on a flat black latex paint might avoid the solvent problem too. (Or very light spray coats spread out over time.)