If you are worried just about the abuse that it will take, I have read several testimonials to the durability of the ~4" black lawn edging. Lots of people have hit curbs, rocks, road debris, etc. with little/no visible damage. They cut out the center some to allow more air under the car, and have had good success with them. A couple advantages, the lawn edging is readily available at any store that carries gardening supplies (Wal-Mart, Lowe's, Home Depot, any garden center, etc), and since it's the end of the year, you will get what you can find for even less than the normally low price! I bet you can do your mod for less than $10, and have enough left over to replace the whole thing if anything would happen to it.
Here's a link:
Home Depot Air Dam