Product review (4 stars): Long story short - I used the vent flap from this one on the existing cover and all is fine.
I had mistakenly ordered a 6 inch cover previously based on the dimension of the cover versus the diameter of the pipe it is to cover.  Also - I was hoping for something that was flush on the back similar to what I had. This has a one inch deep protrusion from the back meant to fit around the exhaust vent.

What I found was the one I had previously also had at one time a protrusion but it had been cut away by the previous installer. Long story short - I used the vent flap from this one on the existing cover and all is fine.