Yes, welcome to ecomodder.
What is the use case, IOW will you be hauling liquids (easy aero), books, or grain (still not bad) or refrigerators (do the math)? If you carry through with the underside and wheel spats this could be an excellent base vehicle, but it carries a lot of cruft between the wheels.
About the lack of plan taper: Air has both a pressure and a velocity (relative to the vehicle). Because of this air moving down the flat sides at the same speed as the air moving down the sloped top migrates upward. If it encounters a sharp edge a vortex forms, tiny at first, but growing into counter-rotating dust devils following behind the vehicle. That's why the Template is a half body of revolution.
Would your truncated rear be a rectangle or a half-circle?
Here is an ordering of similar shapes. Circle=>
squircle=>square:
...and something for inspiration:
