Are you thinking in front of or behind the stock grille piece?
Does that piece normally block ~30-40% of the opening? Remove and replace with perf metal with the same blockage. Back it with a second piece that can slide 1/2 the distance between the holes. Depending on the hole size it might go from fully unblocked to some % blocked.
It can slide side-to-side or up and down. Manual cable or small electric stepper motor.
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