Lexan is a trade name for polycarbonate. Some other trade names are Hyzod, Tuffak, Makrolon, IIRC. If you use polycarbonate, look for UV resistant types. 3/8 and 1/2 inch stuff is mighty tough stuff.
I am also a fan of UV resistant polycarbonate for Windows and such on boats. You could also use cast acrylic, while not as tough as polycarbonate it is inherently more UV resistant. At the thicknesses mentioned, either would be fine in a normal window sizes found on most 25 footers I'm familiar with.
dj