[NeilBlanchard]

The square one would definitely higher Cd. When you include the ground plane (and the wheels) in the picture, that changes things a lot -- the air flow is not symmetrical. That is why the template is a half revolution.

The frontal area is a separate issue from the Cd -- greater frontal area increases the CdA, but it does not change the Cd, per se.

Edit: I may be misunderstanding your question. All three of the shapes you are proposing are "flat bottomed", right? Elsewhere in this forum, someone did a CFD study of the round shape with a slightly curved underside (I think, and air comes out from under the bottom an spirals up and around the sides, if I recall correctly? So, the template is a general guide for shaping the upper side, but the interface of the wheels and the ground with the car chassis have to be worked on more.

That Boxfish model had an amazing Cd of just 0.095, if I am not mistaken.