You can make a functional under panel out of any material that is strong enough. I've read an article in a Japanese car magazine, in which they tested under panels made out of several different materials like sheet metal (aluminum, or was it stainless steel?), plastic, and even plywood and they all yield the same result (when it comes to lessening turbulence under the car). I can't remember exactly how they conducted the test tho. Anyways, with that being said, you probably want to stick with metal or CF because of heat and other stuff. (plywood will rot and you don't want to drive around with rotten pieces of wood under your car!)