I've spent some time looking at the rear bracing in my Dart and ways to improve it.
Just by grabbing the diagional braces, or just tapping on the package tray you can 'hear' improvements are possible.
At the time I just rolled some edges and added some longer welds in addition to the stock spot-welds, only to think to reinforce this area on a later date when I 'got a plan'.

As said, a simple (beadrolled) sheet of metal welded to the existing structure would moet likely add a significant amount of strength to the area (and at the same time block off any exchaust fumes that might find their way into the trunk area.