That back is like a "window frame" for the film holder to rest against. It provides 1/2" of support on all four sides of the film holder. Does that make sense? The first page also mentions that you can make a second one to press against the film holder from behind... that would be for applying some pressure to hold the filmholder tightly in place. I haven't done this myself so can't offer any other comment...

I really like the wide angle field of view on my 8x10 camera with a 5" "focal length". The equivalent on a 4x5 would be 2.5 inches. Others might have a different idea what a nice focal length would be.

In re-reading this thread I realized I might mention something for future readers. My 5x7 camera has a mat board "window" that holds the paper in place on 3 sides. That seemed to work pretty well. But when I did the same thing on the 8x10 camera, the paper fell out inside the camera. Now it is is held in place on all 4 sides by strips of mat board about 3/8" wide. Even this is not ideal.... I've noticed that the center of the paper still has a tendency to warp out and I don't get nice straight border lines, but slightly curved ones. I don't own any 8x10 film holders, so I don't know how they manage to keep the film flat, but I think Joe's suggestion of a couple pieces of two-sided tape, not too sticky, is a good one and I will try it soon myself.

Have fun!