I've read that paper now. The most interesting thing is this:
The depth of an unventilated box cavity for lowest drag was 0.37 * D, where D was 'effective body diameter'.
If we take that as an average of height and width at (say) 1600mm, that would suggest a cavity depth of 600mm is best!
But, with the cavity ventilated (only on the sides and top), the equation drops to 0.27 * D, which would give a depth of 430mm.
Obviously, do your own calculation of 'D' for your car.
Note that the testing was done on only a scale model squareback 'shape', not a real car. On the shape, the best reduction in drag was up to 10 per cent - substantial.
For reference, the paper is: SAE 2012-01-0171,
Bluff Body Drag Reduction with Ventilated Base Cavities
I don't know what the above depth was, but nowhere near the 560mm I calculate from the above equation (400mm with vents).