I was waiting for the cold weather to get here to do this. (Ninja Edit - Coroplast was from "recycled" campaign signage.)
Hopefully be able to leave a complete grill block on all through winter. I used a combination of Liquid Nails, Silicon Caulk, and Zip-Ties for attatchment. Liquid nails to hold the blue foam in place. Silicon for sealing where the Coroplast meets the bumper as well as sealing up the corrugation on the Coroplast so that no debris (water, salt, dirt etc.) gets into the hollow inside of the corrugation. Zip-Ties for holding everything in place while the liquid nails and silicon dried.
Removed front corner lights. Removed the front turn signal markers.
I skipped some pictures of the general removal of the bumper but did have this picture. Red arrows indicate bumper screw, and white arrows indicate hood latch mechanism screws (locations are approximate)
The space that was being blocked was a bit time consuming to get the proper blocking for because I wanted a flush mounted grill block rather than a recessed pocket that would create more drag. The bumper also has a curve to it horizontally so that had to be accounted for as well and the vertical differences in the depth of the bumper.
I used blue insulation foam to space the Coroplast so that it would be flush on the front.
From the back :
This the the middle where I will cut out some of the Coroplast if I encounter any overheating issues. Used a heated up coat hanger (soldering gun would work too but I don't have one) to poke nice clean holes in the Coroplast for the Zip-Ties rather that poke with a knife.
Used silicon caulking all the way around as well as on these corner "wedges" to hold them in place.
I did my best to get both corner wedges the same size but that didn't quite work out.
The final piece installed. Clear caulked around the zipties.
Before (best picture of my personal car I could find) :
Actually found this foam in an alley next to where I work I had had it in mind to use this type of foam because it is easy to shape and cut. I imagine you can buy it at most hardware stores or Home Depot etc.
After such a clean job, it would be great if you could fashion an opening with a sliding shutter whose size you could adjust depending on engine temp. Make the movement by laminating (not sure of the glue to use) and overlapping pieces of the same material, and having it slide make it less prone to move with the force of the wind.
Phase II would be if it had a cable to enable you to adjust it from the driver's seat.
Phase III might be an automated motor that could adjust itself in realtime.
(edit: drawing added)
Reality is merely an illusion, albeit a very persistent one. - Albert Einstein
After such a clean job, it would be great if you could fashion an opening with a sliding shutter whose size you could adjust depending on engine temp.
I've got "the poor man's version" of your suggesion on my grill block. What I did, was make my grill block out of three pieces of coorplast. The left piece and the right piece are held in place by strong "zip ties" (i.e. "permanently attached").
However, the "center piece" of my grill block, is held in place by sliding it in behind the edges of the left and right pieces. So I can easily pop my center coorplast piece in for the winter (like it is now), and just as easily slide that center piece out when it gets warmer (all without tools, as it really does just "slide in" and hold via friction). So with this setup I can add/delete that center block piece as often as I like...
NOTE: This reason this works, is because the center piece is about 2" wider than the center gap, and the zip ties on the two side block pieces don't go up to the edge of their pieces. So you have room to just slide the center piece UNDER the edges of the two side pieces, with enough "overlap" for friction to hold it in place.