As far as a buck for the windshield, I recommend using green flower foam. Press it in place from the inside and use a sanding stick etc to shape it. It shapes really easy. Then I have been using automotive glazing putty over the top and sanding that smooth. Use a high build sandable primer, I like Plastikote, and sand smooth. All imperfections will show, so use a tack cloth to clean the dust. If you cover all sides of the foam with putty, you should be ready for the vacuum forming. If you cant get it to work, your out 6 bucks. I plan to add one more step, covering the putty with a mix of epoxy and microballons. I dont know if the epoxy will react with the flower foam yet. If it doesnt, that would eliminate the putty step. I used a similar technique to make vacuum formed wheel wells for a pickup bed.
Hope this helps, Pete.