TJon...I'll get it to you later today....
Yes vendors sell them, but try and get one for a print you have cropped to an unusual/custom size.
Using foam core is great, the only couple issues is covering the edges...when we used Foam Core we would take Artist Colored Paper Tape to wrap around the perimeter.also Foam core would warp after awhile or sag into the box if they were large. Things tend to ward toward the side not supported or covered, if you decide to use foam core, coat the back side with a sealer. It helps stabilize it and it will breath the same as the front side (which is in a sense sealed by the dry mounted print). This is true with any substrate.
We too would drill 4 holes in the back then use a keyhole router bit to complete the hole for hanging. They were also great as free standing with a slight weight placed in back on lower part of frame. These were very popular in the Parson table days. Everything was flush and cubical. There are so many different concepts, building methods etc.etc. The great thing was is was usually made from all scrap materials heading for the trash bin.