Someone did exactly what your're asking about in my previous car (w126) before I owned it. They removed the entire first aid kit/storage shelf, cut into the sheetmetal underneath to fit the contour of the subs, and at some point (maybe after removing them) covered the whole shelf with a piece of plywood type material covered in red cloth to match the interior. It took me awhile to realize that it wasn't stock.
I benifitted from their installation due to the LARGE (most of the rear shelf) port it created. My 10", bottom of the line, 10 year old Fosgate sub sounded like it was pushing 200w rather than 100.
I'd post pictures, but I no longer own the car.