You could try leaving the hatch glass in place, but removing the rear spoiler (plastic piece at the end of the glass), and use that area to attach a small kamm or extension.
Supposedly sharp edges are better for the back of the car, and the Insight has a rounded rear bumper. Personally, I think it's a great looking car from every angle except from behind. Would be nice if there were a fiberglass or plastic aero rear bumper "body kit".
EDIT: This is a good image to show what I mean. Newer Prius has a sharp edge at the back to improve separation and reduce turbulence.