Product review (4 stars): Does the job - with a little assistance
It is just a bit thicker than a paint tray liner and when extended, you will probably have to tack or tape down the outer edges to keep the air flowing out of the end.  I think for the price, they should have produced a thicker, studier plastic.  But it was all I could find and it does the job.  It does include a small clip to keep it on the vent cover, but comes off easily, so you may want to reinforce that as well.  I use it under a sofa.