Product review (5 stars): Great Friction Fit Damper
These things are great for inserting into the bathroom exhaust fan duct just before the duct attaches to the fan housing.  The dampers in most bathroom exhaust fans are weak.  Tip: (be sure you are using an insulated flex duct in colder climates and you should never discharge the bathroom exhaust fan directly into the attic space no matter where you live)