Cast Iron Round Floor, Ceiling, or Wall Grates for Air or Heat Vent. Register Cover Without Damper, 9" Hole Size, 11" Overall Diameter (ZM-IR-902)

ZM-IR-902  Round Cast Iron Grille 9" Hole Size Without Damper; 11" Exterior Diameter

Made to fit into a 9" 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.