Hummmm.... Either option would restrict the use of the roof! The trunk lid is opened with hydraulics and extra weight might cause a system failure (I'm not an engineer...). The roof is probably not rated to handle extra load either (mine moves with a bumpy road).
An option might be a trailer hitch (UHaul has one part number UHI36980 ) and flat hitch wagon
http://www.sportomotoring.com/dee-zee-hitch-wagon.html
An option might be a trailer hitch (UHaul has one part number UHI36980 ) and flat hitch wagon

http://www.sportomotoring.com/dee-zee-hitch-wagon.html