First off, I would say to fully block your upper grill am the 2 lower outside openings. Then the easiest way to do an auto adjusting grill block for the larger opening like I did with coroplast and and a few metal rods. It works on a counterweight system to allow air pressure at higher speeds to push it closed, then as you decelerate the weights overcome the air pressure and it opens back up.
Here is the thread, if you're interested. The nearly finished product starts around post #38
Idea on Auto Adjusting Grill Block
