I've used polycarbonate, acrylic, and hobby plywood. Velcro buttons can be used to keep the barrier in place, or some camera backs will have
some kind of other feature to keep the protective piece stationary. I wanted to do something a little cooler looking for my 8x10, so took some
copper-clad PC board - thin, strong, and very stiff - and did a swirly true verdi gris color treatment on it. Guess I "spit-toned" it!