22" x 26" or 26" x 22" Cube Core Eggcrate Return Air Filter Grille for 1" Filter Compatible With Duct 12in then you'll need a 14-inch

then you'll need a 14-inch round vent

+ Product Details Size - 32"w X 36"h is the size for the duct opening measurement only

The Contempo slotted Grate

+ Product Details Perfect Fit & Easy Sizing - Designed for a 30" x 34" OR 34" x 30" duct opening (measured at the opening)

Insulation – A thick blanket of fiberglass insulation provides you with an R-Value of 4

22" x 26" or 26" x 22" Cube Core Eggcrate Return Air Filter Grille for 1" Filter Compatible With Duct 12in then you'll need a 14-inch+ Product Details Perfect Fit & Easy Sizing Designed for a 22" x 26" OR 26" x 22" duct opening (measured at the opening), with actual outer dimensions of 24. 5" W x 28. 5" H. Please check the measuring guide to ensure a proper fit. Filter Size 21. 75" x 25. 75" x 1" Premium Airflow Efficiency High performance cube core eggcrate design allows 95% airflow efficiency, reducing energy costs and maximizing HVAC system lifespan. Durable, Rust Free Aluminum