Just to give you an idea...
With the targa panels in the trunk there is still room for one cabin trolley "under" them (they sit 45 degrees with he lower part facing the outer part of the can, cannot explain it better), two six pack of 1.5l water on the side, and another duffle bag on top of the panels. With some goodwill a couple of small backpacks (like for the laptop and so on) as well.
The only painful part is if you decide to put the roof back on in the middle of the trip. You will look like a magician taking out neverending things from the hat.
