Take care! The fixed panel between the rear window and the tail end provides LOTS of strength and rigidity to the rear of the body.
While repairing the rear panel we removed this bit of sheet metal to get some access and had to weld a bit of 1" X 1" box-section across the bottom of the rear window to keep everything square.
The whole rear end of the body got very wobbly. So it was all repaired and welded back in.
I guess if the whole area under there was boxed with sheetmetal, right down to the floor and a front bulk-head put in between the rear wheel wells it could be strong enough to leave a hole in the rear, behind the window, for an opening trunk lid.
I believe that the original design envisaged a lid here but the final design went "solid" for rigidity.