Beige Metal Thermostat Guard with Solid Base - Tamper-Proof Cover, 6-1/4" x 4-3/4" Supply Grille then you'll need a 14-inch

then you'll need a 14-inch round vent

actual front end outer dimensions are approximately 18 5/8" width X 27 5/8" height

+ Product Details Size – 12" x 12" is the size for the duct opening measurement only

Actual Filter Size – 25 3/4"w x 27 3/4" x 1"

Beige Metal Thermostat Guard with Solid Base - Tamper-Proof Cover, 6-1/4" x 4-3/4" Supply Grille then you'll need a 14-inch+ Product Details Heavy Duty Steel Construction Made from 20 gauge steel for superior strength and durability. Tamper Proof Design Provides robust protection to prevent unauthorized access and ensure consistent thermostat settings. Secure Locking Mechanism Features a permanently attached eight tumbler lock with a supplied key for added security. Solid Base for Stability Built with a solid base for reliable mounting and stability. Universal Fit