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Question: why are you so set on building something before doing any testing?
I ask because it sounds like you're fretting over details of the build without knowing if it will be worth building anything at all. You haven't mocked anything up to tuft test, pressure test, or throttle-stop test. Everyone here telling you what shape to use and what details to consider is shooting in the dark. Worrying about materials puts the cart waaaaay out in front of the horse.
If I were you (and I'm in the same situation, since I'm laying out a plan right now to reduce the drag of my truck), I would start gathering cardboard, figure out a throttle stop and check to see that it will work (test windows up/windows down), jot down ideas for devices/shapes to try and how to test them. For example, one of the alterations I want to try is a front air dam. I saved the giant cardboard box from my new refrigerator so I can cut it up and tape it in place temporarily. I laid out the limiting design goals (preserve a front approach angle of at least 20 degrees) and calculated how deep the air dam can be to meet that goal. When I actually test it, I'll start with it at its lowest height and then progressively cut more of it off to see how that affects drag. Then and only then, I'll have the data I need to build a permanent air dam because I will know what height it should be for maximum drag reduction--or not build one at all if it turns out not to reduce drag.
On you Civic, I would build a cardboard back that I could trim to change its angle and then test it. That way you can find out: 1. if such a change reduces drag or not and by how much and 2. what angle/shape reduces drag the most, if any. If it turned out that it reduces drag, then I would worry about what to build a permanent version out of.
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