Well, I went ahead and ordered a used driver-side auxiliary radiator air duct from Ebay. Not sure if I'll use it, but it was only $59, and I can always sell it if I don't. It's a little dirty, but should be easy to clean up. Plus, they're extremely hard to see behind the bumper anyway.

I also found some driver-side auxiliary radiators on Ebay, but they were $250, so I held off until I decide what I want to do...