Why not shape a hard balsa block and shape over that instead? You don't want to shape your lens using Lexan/polycarbonate, the forming temp for a 3D pull is around 390F. with a forming duration window of only around 3-5 seconds. Use something like PET-G, much nicer to work with, just google PET-G sheet and you can learn lots about it. Best of luck to you.
Bob