I installed 3/16" Lexan from local homeless depot because that's the thickness that was in there.. I think 1/4" will tighten up in there better because the 3/16" clamped okay but not real snug. 1/8" will be a very loose fit.
48 in. x 96 in. x 0.187 in. Polycarbonate Sheet $130 #206792207