Everything I know about aerodynamics I learned on Ecomodder.
To me a box cavity has always been a fast and dirty boat tail. Not perfect, probably not good looking, but effective. I've seen some graphics of pretty ones that just needed some skin to be real boat tails, but even one panel hanging out back coming close to the template is good.
I can't find the post, but this was on an early version of The Anal Probe:
The setups on the backs of 18 wheelers these days are a good example. Crappy boattails, but way better than the unmodded truck:
It's not pretty, it's not optimal. It's a box.