covering air holes for sound reducing

how to easily cover those air holes (looks like this to improve sound reduction? I also plan to rubber seal the window gaps below, but the biggest culprit is obviously these holes. Would a dense foam like acoustic foams attached to a cardboard/Styrofoam help? Ik sounds dumb but I need a cheap way to cover it without drilling any holes. Any help appreciated.Thanks via /r/DIY


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