Product review (2 stars): I Don't Recommend It Unless You're Handy With A Glue Gun and Magnets
It does what it is intended to do when you can keep it in place, otherwise I wouldn't squeak out a second star.  The two layers of plastic "vent" material don't actually connect together.  One just slides over the other.  The piece that goes over the floor vent doesn't attach to the floor vent.  It just sits on top of it.  Out cat likes to crawl under the couch, constantly knocking it off the floor vent.  With a glue gun and some magnets, I'm hoping to correct the very poor design qualities of the Dundas Jafine EXTVENT Extend-A-Vent.  If I'd have known about this beforehand, I've not have purchased it.  I've had just spent the time and $10 for materials at the hobby shop and made my own.