Product review (5 stars): Highly Recommended!
I purchased this to replace an old shutter style cover. This is a far better design and definitely seals when not in operation due to a small magnet located at the bottom of the housing/face which aligns with a small round ball (size of a small "bb") on the bottom of the outer flap. Wind proof too! Recommend using 4" aluminum duct pipe. The knurreled end fits inside the opening of the vent hood. The pipe may require some messaging to make sure it doesn't interfere with the inner flap. Once pipe is connected and the inner flap operates freely, I used the aluminum type tape to seal the pipe to the vent hood flange. Silicon caulk would work too. Tape is instant and seals very well.
Note: using an 1/8" drill bit, pre-drill the 4 holes before you do anything. Once the vent hod is in place then you can screw it fast to the structure and caulk. Be careful not to over tightened the screws because you'll distort the plastic and the flap won't seal, or crack the plastic then you're proverbially screwed.