Product review (5 stars): Wekk made and easy to install.
We bought this to install a microwave/hood into a butler's pantry where there had never been a hood.  It was on an outside wall, so it was easy.  Mount the microwave after cutting the proper hole in the wall, and then install this from the outside.  It mated right up to the damper on the microwave and installed easily.  We caulked it for a weathertight seal.  It adds another damper and a 1/4" mesh bird screen.  Good stuff.