Lexan is actually plastic. It is worked, cut filed, sanded, easily with woodworking tools, best when warm. You can get it to pretty much whatever shape your going for, meaning you can bend or contour by selectively applying heat. For qtr windows you don't need much if any contour though. use your glass windows as a pattern, cut your shape with a jigsaw, drill your holes and install. Leave the liner on the sheet when working it or mask it to avoid scratching cause it will easily. Like anything good quality lexan is not cheap, I don't know what it actually goes for retail, but if you know anyone in the sign business or can find a small signshop they will often have end cuts or can get it for you. The only problem with them getting it is you'll probably have to spring for a full sheet ($250?). if you can get some endcuts get an extra to practice on. Good Luck!
