Product review (4 stars): I think the outer width is around 12" and it worked perfect on our siding with some silicone to seal it ...
Fits the straight end of an 8" duct, used a screw clamp to hold it tight.  I think the outer width is around 12" and it worked perfect on our siding with some silicone to seal it up.  See how long it lasts here in nd.