Here we go, another reference for the box cavity (which has come up in other threads, but also seems relevant here): J.D. Coon and K.D. Visser "Drag Reduction of a Tractor-Trailer Using Planar Boat Tail Plates"
(Google Books) The Aerodynamics of Heavy Vehicles: Trucks, Buses, and Trains, Volume 1, edited by Rose McCallen, Fred Browand, James Ross, p250
Quotes previous research showing poor results from splitters and vanes, good results for a 'non ventilated cavity', lots of supporting references given to follow up on, including early development, patents, investigations and optimisation by NASA and an investigation on minivans by the well-regarded Hucho, as well as detailed optimisation info for the tractor-trailer case.