Your using lexan to build boxes? I almost can guarantee it was the lexan that warped.. Did you leave it outside? Was it left in the sun? Most plastic gets soft at 250 to 400 degrees but I could understand it warping over a long period of time.
Yes, lexan used for the top sheet. Yes, it was left in the sun. Yes, it was left outside. However, I've read on multiple locations that lexan is an acceptable material second only to HDPE.