I moved my lic plate down so it would cover the middle of my lower grill, then i cut a 1liter plastic coke bottle from top to bottom. Made some cuts for the grill struts and wedged it in there with the rounded part to the front. I have not painted the half bottles yet but that would be a waste of time. Rather than have a flat surface thought that the rounded part might help with reducing the air pressure on the front.
though it is hard to see (if not impossible) in the pic. my grill block actually curves from the top to the bottom. it is slight but it follows the curvature of the vents behind it. also, there is a seam going down the middle which follows the point on the hood and bumper.
Be the change you wish to see in the world
I suspect that Plasti-Dip spray in white or flat black would be great for a lot of grill block applications. It's a liquid-applied rubber, originally meant for replacing torn rubber handles on metal tools like pliers, now available in a spray. It's very popular on golfmkv.com where people like to do their whole grille with it just for aesthetic purposes...
You can get it at Home Depot/Lowes, Harbor Freight, and so on.