Heres what ive done to two of my family's cars. Sorry about the night pics, its an insomniac thing.
what i did here is cover 3/4 of the round oval. I used some garden edger i found in my garage that has been sitting there for year. I zip tied them on so its easily reversed. I didnt cover that slit, or the bottom opening because I didnt want to cover the trans cooler.
Whats not visible is is that i replaced the radio antenna with a short bolt. lets see if that does anything.
what i did was cover 4 upper grille slots (i didnt want to over heat since the fan is mechanical) with packag tape laminated shoebox cardboard. I also covered the lower grille (only 4 again) with just packing tape.
and thats it.