One thing that can help to prevent cracking in both Plexiglass and Lexan is to anneal each piece after ALL the machining (holes drilled, edges rounded, everything) is completed. Preferably, if the piece is to be bent into place (as it often is when installing these sorts of windows), this would be done with the piece clamped against a "mold" of the approximate curvature as the finished installation. This will do two things: first, it gets rid of most (ideally, ALL) of the micro-cracking induced by machining; and second, it dramatically reduces the stresses (and thus the strains) imposed on the material when it is installed.
It's in the lazerette?
Oh, never mind...
I didn't REALLY need it anyway.