The Anal Probe is legendary.
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Not quite. It has a flat spoiler, Bonneville style, and a difusser. Race cars are built to the rule book, which is why (I suspect) the endplates are rectangular and that size.
I question the validity of the difusser. It had a slot opening to the front that was maybe 1/6th or 1/8th the opening at the rear. In theory, that would expand and slow the air reducing the base pressure. (aerohead?) I think it just increases the area the base pressure is sucking on.
If the rear edges of the spoiler and difusser were aligned vertically, and a small fillet was added to connect the two, that would constitute a box cavity. But that might not fit the rule book. And it would be a fairly inefficient one, as a box cavity is supposed to step or curve inward to fit The Template's line.
Here's another picture from Bonneville. (Note the similar sized endplates) The sides of the difusser are curved and it's open on the bottom.
I think that would be better. Here's another example with fences:
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As for the 'bug deflector', the onrushing air pretty much flattens against the frontal area. At the point of the headlights about all you can do it try to divert it sideways. See the Nissan Leaf's headlight and rear-view mirror.