https://ecomodder.com/forum/member-a...ing-effect.jpgHere's a $425,000 box cavity.
https://ecomodder.com/forum/member-a...ion-plates.jpg
All the trailing edges are positioned where the trailing edges of an ideal boat-tail would be.
Between the separation line and the trailing edges, vortices are captured within the confines of the intentional void.
Outer flow follows over the outer 'surface' of the vortex, creating a virtual boat-tailed 'surface' of this 'dead' pool of air, on top and sides. ( the bottom is ruined because of the rear wheel turbulence and DOT bumper.
In order for the vortices to be captured, the rear edges of the cavity must be located at the 'streamlined' position.
* Your top is fine, it's a flush continuation and extension of the profile.
* The bottom seem okay as well.
* The sides however, seem too far inset to allow for vortex capture, producing unstructured turbulence instead.
I've added W.A. Mair's boat tail. It's outer contour would illustrate where 'edges' should be positioned.