The #507D Starting Collar w/Damper for 1" or 1 1/2" Ductboard is available in 12 sizes from 4" through 20".
M&M Ductboard Starting Collar With Scoop and Damper, 507DX Series, 6 in Fitting, Round Shape, Steel, Finish: Galvanized, 6-1/2 in Height, 30 ga Gauge