sorry for the zombie thread, but I have a method which I intend to try for this.
Using silicone caulk and corn starch, somewhere between equal parts and almost no starch (more starch cures faster and makes harder but less flexible end result). You mix the silicone and corn starch together in a big plastic bag (gallon size or so, wont stick to the bag) and then when its nice and mixed, roll it smooth with a rolling pin (still in the bag, no mess). you can have a book of the thickness you want on either side, and it will end up with a large angled slab of silicone when it cures. peel the bag away, and cut to fit. probably would want velcro to attach, but you could also embed something in to fit into holes in the side of the bindings too, like a thin frame.
anyways, it should leave a silicone wedge that would work perfect for this.
you can color it with linseed oil paint from an art supply store, for crazy looking bindings