The Cd on the newer metro sedan is better than that of the hatchback, I'm assuming due to the smoother airflow off the back due to the trunk, a fastback like this would benefit it even more. Plus it would have a better attachment point for the farther back portion than a cammback or boattail on a hatchback
Getting into the trunk is what the folding rear seats are for lol. you could just do the whole thing out of plexi, and make a hinged edge type frame that attaches at the top of the rear window...