Bestvents 510WH
Grill with a flange and (4) screws to hold it in the foundation opening, has no flap or slide mechanism to open or close it, so the air can constantly move through. Size of 7-3/4 in x 15-1/4 in so it easily fits the standard 8 in x 16 in block foundation opening.
California residents see Prop 65 WARNINGS