[aerohead]

The original box cavity is related to Continuum Dynamics and testing done by NASA at the Ames Research Center, Palo Alto, California, under an SBA grant from DARPA. Designed for an 18-wheel tractor trailer van.
The Audi structure would qualify as a box-cavity also.
What Mercedes-Benz and Julian Edgar did is more a 'hollow' FKFS 'Kamm' extension, or 'elongation' in Hucho-Speak.
Basically, the box-cavity's panels are inset from the exterior perimeter at the attachment point, to such a degree that the point they project to falls along the imaginary line of W.A. Mair's boat-tail profile, top, sides, and bottom.
Flow separation occurs at the trailing edge of the vehicle, then reattachment right at the end of the panels, capturing four, locked-vortex, stagnation bubbles of circulating air, over which the outer flow will skim across, almost as if they were a solid boat-tailed surface, separating at the 'new', smaller cross-section.
You must pay for the vorticity, however, the pressure regain, and reduced wake size, conspire to increase base pressure and lower pressure drag, just as Julian describes.
They're light compared to a authentic boat-tail, much easier to fabricate, with less material, and with much of the good. Today's 'Trailer-Tail' is a deployable, hybrid version of the technology, more like a boat-tail, but also hollow, unlike any of the FKFS tails of the 1930s.