I used this closed cell soundproofing foam in my plane and I don't see why it wouldn't work as a great wheelwell liner....definitely tough, waterproof and fireproof. It's sold by the linear foot in 48" wide sheets and available in several thicknesses. Of course you would want to make a paper pattern and cut to fit, but it's not a big deal; cuts easily with a utility knife or heavy scissors.
http://www.aircraftspruce.com/catalo...oofinstall.php
Definitely cheaper (and possibly superior?) to some of the products discussed so far.
Rodger