Here's what I recommend and use on my Cobra. Looks good and is cheap. Your local hardware store may have something also and I think it's called, heavy duty pet screen's for doors. (I live close to speedway motors, so I picked it up)
http://www.speedwaymotors.com/Radiat...reen,3546.html
Radiator 31" x 19" Shaker Screen, Plain Steel
Item #: 91734220 Price: EA
Save your radiators from rocks and mud with this steel shaker screen. Measures 31" x 19" and can be trimmed to size. (Hardware not included.)
OR-
This is what we use on the sprint car.
Honey Comb Radiator Protector, 15" x 22"
Item #: 91734213 Price
Nomex honey comb. The same stuff the Outlaws use. Lightweight but very strong. Place directly in front of radiator and let it take the beating instead of your aluminum radiators.
Some guys have raced a whole season with one honey comb. Usage will vary depending on track conditions. Can easily be cut down.
Great for street rods as well!