I sell this stuff so I know "a little" bit about it ...

1) Forget the heat gun. Unlike plexi, polycarbonate (Lexan) has a lot of moisture in it. If you heat it up enough to bend it you will get so many air bubbles in it you won't see out that area. Polycarbonate can be cold-bent in excess of a 90* bend with a corner radius its own thickness ... getting it to take the shape of your windshield shouldn't be an issue at all. Cut it, lay it in place, fasten it down.

2) minimum 3/16" for a windshield

3) I suggest Makrolon brand - its less expensive and clearer than Lexan (Dow Corning brand name). If you can find it, buy "abrasion resistant" material - usually only coated on one side so you want that side OUT.

4) clean with MILD detergent ONLY - no Windex and especially NO SOLVENTS OF ANY SORT.