My Civic has decorative vents on either side of the lower grill. I think on other models, they might have lights there, I don't know. The only go in a little over an inch maybe.
Here is a Civic with a similar byumper to mine:
Would there be much if any benefit from covering those? I was planning to take a piece plexiglass, and cutting it to fits over the vent, overlapping the sides a little. I would paint it black and have it held in place by double back foam tape.
I was also thinking about extending the plastic piece in about 4-5 inches to overlap the sides of the lower grill to make a partial grill block.
Another idea I had was to make an auto bra that would cover the faux vents, and part of the lower grill.
Waddya think? Will it make a difference?