Adding Weight or ballast in the trunk.

I'm pretty sure the rule book gets pretty specific on the box construction and a "by-the-book" tech guy wouldn't allow it. The two little flip latches likely wouldn't contain the ballast in a violent crash and the wall thickness is probably also be a factor in containing the contents as well as keeping it secured through the floor and frame.
Youre right, wall thickness isnt close to being legal.
But then my car isnt subject to divisional/national event tech inspection. If the techie ever disallows it, i can remove it.