Cast Iron Round Floor, Ceiling, or Wall Grates for Air or Heat Vent. Register Cover Without Damper, 8" Boot Size, 9" Overall Diameter (ZM-IR-802)

ZM-IR-802  Round Cast Iron Grille 8" Hole Size Without Damper; 9" Exterior Diameter

Made to fit into a 8" round hole or duct, these grates have an exterior diameter of approximately 1" greater than their boot size.  The flange measures 3/16", and is pictured in the side view photo. You may need to modify for different mounting applications.

None better anywhere at any price.