Don't use plexi unless you have some HARDCORE 1" thick stuff. (or much thicker, depending on the surface ara) Lexan's bad too, It's purpouse is to not break, but it still flexes.
Plexi & lexan flexe a whole bunch, and SIGNIFICANTLY lowers your potential SPL and SQ.
See-through sub boxes are for show, not preformance. If you just had to do it, I'd use what they make aquarium "glass" out of, I want to say it's acrylic... but whatever it is, it's damn think, and dosn't look flexable at all to me. Considering the surface area these huge pieces have, a much smaller piece wouldn't have to be that think.
(aquarium glass, as in - the huge WALLS of "glass" with dolphins and whales in the tanks...)