Crude but effective: On my bikes I have always zip tied some 1/8" wire screen over the radiator. Buy it from the hardware store and paint it if necessary to blend in. leave an 1/8-1/4" gap between the screen and the radiator to allow for rock impact. If you do a good job you cannot notice it. Very effective protection.
Roadracing World had a recent write up on how to easily and cheaply make a radiator guard but unless you are riding on the track and following hot sticky debris flinging bikes you shouldn't need it much. But for 10 bucks or less you can build and install a little piece of mind.