It looks like you're new here. If you want to get involved, click one of these buttons!
Honda Odyssey Grille Insert
A number of posters did a home made solution using gutter guard or other screening. There is also a company the makes a custom billet aluminum grill ti fill in the lower opening. Looks pretty sharp. I think the company is called "rm. grille", or something close to that.
of course, there is no definitive answer about blocking airflow, but I guess if the temp doesn't rise, and the AC stays cold, shouldn't be a problem. besides, you can sit parked with the AC on without a problem, and there sure ain't no airflow then!
2013 Acura RDX (wife's), 2007 Volvo S40 (when daughter lets me see it), 2000 Acura TL (formerly son's, now mine again), and new Jetta SE (son's first new car on his own dime!)