The truck picture had a mirror, but the previous BW one's didn't. Its no biggie, it was just an observation....
So on your picture, correct me if any of this is wrong:
The vertical metal is for the rear seat back to mount. The rear deck would be horizontal off the top of that vertical metal. So that leaves something like 3 feet from the vertical metal to the back door when the fiberglass shell is installed.
If 3 feet long by approximately 4 feet wide is correct for that engine cover area. Then that would be a pretty small storage area behind the rear seat. Though it is notably bigger than a regular bug.
Then I would guess that by utilizing the rear seat area for storage, it would be approximately 3 feet long by 4 feet wide, when measuring from that vertical metal going forward.
I'm trying to envision how much interior space I'm actually gaining by having the shell addition. It doesn't seen like its a noteworthy gain of usable space. Though it does make the bug look a lot more appealing.