This return plenum box is designed to be used with a 20"x20" filter grille. The size of the box is 20"x20"x5" deep with a 3/4" flange, and the 12" collar is 4" long. Overall dimensions to the outside of the 3/4" flange are 21 5/8"x 21 5/8". It's manufactured from 26 gauge galvanized metal. The collar will be cut centered in the plenum box for you to install it. If you need a different size or a different size collar, please check our eBay store. You can call or message us if you need a custom size that you don't see listed. We will make and ship it out to you quickly.

Return air plenum, for filter grill 20"x20"x 5" with 12" collar & 3/4" flange