I think your last car looks positively excellent, actually
. I really struggle to see anything fundamentally wrong with the design, save for detail work of course. The biggest drag contributions were likely from exposed parts I imagine. I think the car is low enough that most of the airflow was already directed around the car, although at the front there is first a gap and only then a sort of a fence so it is difficult to say. If you manage to get the air around the car rather than underneath it, then the underbody shape of course becomes much less important. I do have to say I like the shape a lot
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Low ground clearance is really the way to go, but if the rules give a minimum ride height then you must try and make the underbody as smooth as possible, as the air is going to get under that, and it's also likely to get accelerated so it is doubly important that the shape is smooth or the drag will be that much larger.
In your current design, one other idea; you could put e.g. sculpted styrofoam sides onto the car that would have more rounded edges, so that the flow wouldn't separate as easily from the sharp edge, or at least that the separation will be reduced. The air will want to go both over and around the car in any case.
