Cast Iron Round Floor, Ceiling, or Wall Grates for Air or Heat Vent. Register Cover Without Damper, 6" Hole Size, 7 1/2" Overall Diameter (ZM-IR-602)

ZM-IR-602  Round Cast Iron Grille 6" Hole Size Without Damper; 7 1/2" Exterior Diameter

Made to fit into a 6" 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.