Home Depot occasionally sell these plastic mesh ppl buys to make chicken coop, you can buy a whole roll of them. I used these for making grills for my other car and they look and held up very well. Bed them in with epoxy, lots of it, and let it cure completely before installing.
You can also use the ones from metrorestyling.com. Pricey but looks pretty clean. I would get it in black so that you don't have to spray paint it.
Universal Aluminum Diamond Mesh Grille