Check the listing for details. 18" Plenum Box for [18" Linear Slot Diffuser - (1 Slot) Single Slot] Premium Ins. Condition: New. Listed at 99.99 USD. Galvanized sheet metal Plenum Box with black foam insulation, ideal for Texas Buildmart 18" lineal slot diffuser. This air-distribution box attaches directly to the supply of the HVAC equipment that heats or cools the air. Connects to: 8" flexible duct in oval shape. Can be used and brazed to a 4" or 6" wood stud. If you do not have this size flex, you can use a reducer to modify the size. Professional HVAC Installer is Recommended.Ideal for Texas Buildmart 18" Lineal Slot Diffuser - 1 SlotGalvanized sheet metal Plenum Box with foam insulationTotal Box Dimensions: L: 19 1/16 W: 3 H: 11 11/16Entry size: 8" flex in oval shapeThis air-distribution box attaches directly to the supply of the HVAC equipment that heats or cools the airIf you do not have this size flex, you can use a reducer to modify the sizeHere's a link to the diffuser that fits this Plenum Box:18" Linear Slot Diffuser - 1 Slot - AC Vent - Register