I feel your pain. Replacing my hard plastic "flaps" has been on my list for quite a while now, although I have yet to replace them. One option is one of the universal fit flaps that they sell at auto stores, which are essentially large pieces of rubber that can be trimmed to fit. I just don't know how well it would work since they would be quite a bit heavier than the stock plastic. As long as the anchor points (existing screw locations) seem strong enough, pretty much anything should work.
These are flaps that I've been eyeballing:
http://www.rallysportdirect.com/Rally-A ... e-Red-Logo
Urethane won't be as heavy as rubber, but should be quite a bit more durable.