Although everyone may not agree, I like to size the sleeve used as a heat shield for the upper so that it just covers the plastic and does not extend upward into the airflow. This allows the top of the reduced diameter stovepipe to be cooled by inrushing air. If you extend the sleeve, you reduce the cooling effect on the hottest part of the system. Also, it is a good idea to keep the wall thickness of the sleeve as thin as practical. That way there is less mass to retain heat against the plastic and a greater insulating gap between the hot pipe and the sleeve.