There are several types of plexiglas or Lucite. What we need for automotive use is UV-stabilized acrylic so the window does not 'sun-craze'. And tightening the mirror-screws too tight will stress-crack acrylics, too. The major gain is, the stuff is cheap: when I made my first set of front quarter windows some 18 yrs ago, I made five of each side. I just installed the last of that 1st batch, each of which cost me $3.