How about killing two birds with one stone. With the boattail concepts being attached to create higher aero efficiency, a viable addition of a swing-away cargo boxes with efficient aero tapered forms can satisfy the requirements. Trailer hitch connections could be made strong enough to carry such aero boxes. The recent iQ's deficient/non-existent cargo capabilty can benefit with such concept. I quickly Pshopped examples.
http://i429.photobucket.com/albums/q...IQpapoose1.jpg
http://i429.photobucket.com/albums/q...IQpapoose2.jpg