Reddit reviews 24"w X 12"h Steel Return Air Grilles - Sidewall and Ceiling - HVAC Duct Cover - White [Outer Dimensions: 25.75"w X 13.75"h]
We found 2 Reddit comments about 24"w X 12"h Steel Return Air Grilles - Sidewall and Ceiling - HVAC Duct Cover - White [Outer Dimensions: 25.75"w X 13.75"h]. Here are the top ones, ranked by their Reddit score.
24"w X 12"h Is The Size For The Duct Opening Measurement Only, Actual Front End Outer Dimensions Are 25.75" Width X 13.75" Height - SEE BELOW IN DESCRIPTION HOW TO MEASURE A GRILLEHIGH QUALITY Solid Stamped Steel Construction With Recessed Screw Holes (SCREWS INCLUDED) To Easily Install On Ceiling Or SidewallQUALITY PACKAGING - Carefully Shrink Wrapped With Solid Cardboard Backing To Prevent Damage In Shipment - Hand Inspected for QualityHEAVY STEEL CONSTRUCTION - No Scuff Powder Coat Finish - Smooth, Shiny, Scratch-Resistant, Beautiful. Looks Like It Was Just WaxedUNRESTRICTED AIRFLOW For Your HVAC System - 50° Angled To Provide Maximum Airflow – ½ Inch Spaced Fins
Lob an inch off the bottom of the closet door, cut and install a vent like a return air vent above door frame, place 2-3 box fans blowing out behind grille. Should move enough air in and out to keep things in decent shape.
Might be able to put screened louvres (sp?) over it. It would hide the actual size of the hole. Would be an epic amount of passive ventilate to complement a roof vent or whaterver.
Edit:
like this, maybe an outdoors one or spraypainted with outdoor paint to match your hoopty.