I just use coroplast, it does sag if nearby heat sources such as exhaust, haven't seen sagging around the radiator. Its waterproof, heat formable, light, cheap, and easy to work with! I found 4'x8' sheets at home depot for $15.
You can see some of the sagging here
Mine is attached with a sh*tton of zipties, lots and lots of screws into the "side skirts", and a few bolts by the exhaust heat shield. Just make sure you do not use zipties to secure your undertray to brake lines or fuel lines. In difficult spots, I used 3M double sided sticky tape with an L-bracket. Still holding up fine.