I did this in the parking lot of home depo so it's a "ruff draft".
I used foam pipe insulation for 1'' diameter pipe (I took several sizes out and 1'' was the biggest and seem to be the best). It was 1.95+tax for 6ft. I used about 2 1/2 for the one you see here, seeing how I only needed half I did it really fast knowing I'd have enough to make it over again a little cleaner.
It comes with some self adhesive ends as well. I pealed them but not having experience I let them touch together on one of them and damaged the adhesive (still didn't come off at 70mph tested speeds).
I call it a "partial" because I am sure some air can squeeze by even though they fit tightly together. I also left a small air gap on the bottom right which I will cover on my next attempt.
I monitor my temperature currently with a obd2 elm bluetooth and torque app on a old android phone, I normally average 191-194 degrees F driving, it does the about the same warm up time before/after and no noticed difference in as of yet (its all the little things that add up). My highest temperature was 203 idling in traffic for over 15 minutes.
I'll try to get a few more pictures of making them and installing them soon, My goal was to block the grill using no major adhesives on the body work, no major drilling/screws into anything and not too much of an eye sore (I would like to find some black pipe insulation).