I'd posted that for the difusser, but for the spoiler go to the five pages here:
ecomodder.com/forum/showthread.php/flat-bonneville-type-spoiler. It provides a surface for any vortexes to reattach to.
Beyond that IIRC someone said the side plates are what they are because the rules dictate a maximum length and height. They just maxed it out and rounded the corners.
A Studebaker
Rooftop fences and the skegs appear to flare somewhat (maybe not).