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The thinner it is the more attachment/support points it will need. Too thin, it may also flutter, compromising some of the benefits, one reason I personally like coroplast. In addition to being light, it has internal reinforcement. I really can't speak to the stiffness of the other materials.
Too thick, it is sometimes difficult to bend as needed, and gets heavy. The area to be covered is the variable. A complete underbelly on that car will probably require more than a 4x8 ft. sheet.
Most of the materials mentioned here have specifications on various internet sites. You can get the weights and various performance parameters. Melting points would be important if you are going to cover exhaust parts. Catalytic converters in particular get very hot.
Sometimes you can make use of "natural" attachment points where a single self tapping screw and large washer will suffice. In some areas, where no further access is required, pop rivets work. In some extremely open areas, you may have to build a lightweight structure as I show in the Echo link. You need to think ahead to how service access will be obtained.
Sorry not to provide a direct answer, but I think there are too many variables to be more specific, and I don't know the underside of the car.
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