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if you can get away with it, one sheet of coroplast (or at least two joined well at the seam) will yield the best results, focusing on getting the leading and trailing edges tucked as close to the front and rear bumper covers as possible. sheets of aluminum can be used to shield the plastic from exhaust pipes, and/or header wrap those areas that come close to the plastic so you only have to cut reliefs where the pipe would touch or come below the plane of the plastic. though, thinking about it, wrapping the whole exhaust might be best so you don't overheat the whole cavity created by the coroplast.
another way to go is just cut relief holes where anything pokes below where you would like the plastic to be, and/or leave a channel for the exhaust. both will compromise MPG and engine load, though.
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