I don't remember where I got the stiff I used on my Nova. Seems like it was a closed cell material I used. I drilled then taped the windshield bed then used machine screws to install mine. The screws are under the moldings so you don't see them. I'll guess I spaced them about 6 inches or so. You can't really tighten them up much or it deforms the lexan and it looks like crap. Might try McMaster Carr.
There is a link to a lexan install we did a few years back in my sig if you want to see it. Prob alittle more involved than what you are looking to do, but you might get some ideas. Good luck with your project.