So I was going to just make this the final shot of my Ross, but I figured my solution might have broader appaeal. I lucked onto the best fender extender material I have used to date; the clear plastic ribbed box cover of new TRP brakesets. It's light, flexible, medium easy to cut, but very tough. It seems to hold just the perfect shape of the fender yet is stiff enough to slough the water off without flapping and letting it sling backwards.
If you went to your LBS and told them you're on the lookout they'd probably hold onto one for you.
[IMG]Fender Flap by mtypinski, on Flickr[/IMG]
Oh, and it is roadtested. Today's commute was a freezing rain/ water/ slush/ ice mess of muck. Got in with a clean drivetrain and shoes.