Cast Iron Round Floor, Ceiling, or Wall Grate Vent. Register Cover Without Damper, 11" Hole Size, 13" Overall Diameter (ZM-IR-1102)

ZM-IR-1102  Round Cast Iron Grille 11" Hole Size Without Damper; 13" Exterior Diameter

Made to fit into a 11" 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.