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The shuttering mechanism will have to fight the air pressure one way or the other, either opening or closing. And you will need space behind the grille to match the depth of the shutters.
I propose two sheets of perforated metal. Our local recycler has a lot of stainless steel with inch-and-a-half perforations. When the holes are aligned you'd have a ~50% grille block, but slide the inner one sideways and it goes to 0%.
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