Product review (5 stars): Works Great!
I bought this years ago - elsewhere.  This sealed our short-run from the dryer perfectly - we used to get a draft through the louvered vent, this sealed it up nicely.  Has held up great, works perfectly, getting a bit "yellowed" from the sun.  Just be sure to clean it out periodically and make sure the internal sliding "cap" is moving freely.  It is kind of big and obvious if it is out in the open, but you could probably paint it to mach the color of your house/building.