Catty

My old attic fan make a lot of noise in windy day. I think about add automatic shutter between fan and existing attic vent. One reason, I live in community and can not alter the look of house. Other reason the existing vent is smaller than the automatic shutter I can find. I am thinking build a box on top of existing vent and install auto shutter inside first and install gable Attic next. It is odd. I never done any woodworking and not sure if I can install fan on a custom box, not install directly on gable stubs? Any suggestion?