I've never really had a problem with puncturing grill blocks (even lower grill blocks) and I've had them made of weaker material than coroplast. That plus the subsequent layers of body filler (bondo) will strengthen it more.
And body filler was the treatment I put on last night. First, I trimmed off the excess fiberglass and sanded down the edge back to where I wanted it to be. I used a dremel tool cutoff wheel and flap sander to speed things up.
Then, the first coat of bondo went on. My wife even took a shot at applying it (since we just got an old truck to restore and she wants to help) and did a pretty good job. I did the other half myself.
Then of course comes the fun part, sanding!!! woohoo... not really. I used a random orbit sander and sanding block (actually a drywall sanding block) to knock it down. The first coat turned out pretty good. That fiberglass mat was VERY uneven and the bodo filled in the lows pretty well. Hopefully one more coat should do it.
At this point, I'm going to say the foam backing is really beat up. Trying to hold thing thing without smashing the foam has been a pretty big pain in the butt. Its hard to apply pressure when you can't hold the back of the thing you're wanting to push on. I'm not sure how this'll effect the end product, but just thought I'd mention it.