Wondering what is good material to use for a grill block. Keep cost in mind. Let me know what you used.
Coroplast, is what many of us use. It's a corrugated plastic (think of corrugated cardboard, but instead of paper it's made out of lightweight plastic), often used by sign makers (as it's reasonably cheap, water proof, and lightweight). The reason the stuff makes good grill blocks is that's it's reasonably strong (due to its corrugated internal shape), lightweight (i.e. doesn't add much weight to the car), water proof (since it is plastic), and easy to cut and bend as needed (so it's easy to mold into the shape you need, and also easy to put holes/fasteners into to properly hold in place).
You can buy the stuff from sign makers or from suppliers of sign makers (I mail ordered several black pieces from a sign maker supplier when I got my supply), for reasonable prices. Or, if you really want cheap (and you don't mind painting them first), you can just wait for the political season to run its course, and then ask any neighbors with coroplast (political signs) if you can have the signs after the campaign is over. Likely you can find your coroplast for "free" that way...
NOTE: Not all political signs are made out of coroplast. In fact, many of them are made out of either sheet metal, or simple sheet plastic. So you have to look carefully at the construction of the sign. However, many of them are in fact made out of coroplast (corrugated plastic) and would work fine for a grill block (as long as you paint them first). And the way to tell is just look carefully at the sign construction. If the edge of the sign looks like a (corrugated) cardboard box, but the sign isn't made of paper, than the sign is most likely made of coroplast...
I used the foam backer board material that is used behind vinyl siding, wrapped in duct tape, for grille blocking that went between the decorative upper grille and the frame behind it (see my grille blocking thread if you want more detail).
I relocated my license plate to cover part of the lower grille for a stealth grille block.
I also moved the front license plate down to cover the lower grill and used some zip ties to connect it to the horizontal grill slats. It is super stealth and I like to think that it gives the appearance of the car being lower.
For the rest of the grill blocking I used some old hockey tape. Not really sure if it blocks out all the wind but I like to think it does something, they do it in nascar but who knows what type of special tape they use. I used a clear tape that is made of a more plastic like material for the bottom layer, and on top of that I put black tape that is more cloth like. It has held up nicely for about a month and wasn't affected by heavy rain.