i use this:
search e-bay for:
Red Transparent Acrylic Plexiglass sheet 1/16" x 5.5" x 5.5" #2423
Its like 4 bucks and enough for 2 Its a hard acrylic, but thin enough to be able to flex just enough to fit the armor. best stuff ive found. they sell larger sheets as well that i use if i have to do multiple builds, but if ya dont need it, this piece is perfect.