Product review (3 stars): Draft Blocker for Bathroom Fan
I bought this for my bath fan because the other one broke.  This item is made from light plastic so I was fairly gentle with it.  I had to spelunk into the attic and fend off the giant spiders to install, but it went together fairly easily.  Clamp was sold separately.  Now the draft from the attic down into the bathroom has been cut off.  So far so good.