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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 (daughter stole that one), and 2000 Acura TL (formerly son's, now mine again)