The Nissan delta wing nose is un-necessarily long. If it was blunter you could meet the 16° approach angle with a lower stagnation point.
You keep reverting to that high nose. It needs to be like a Porsche 356 with a baby buggy bumper. Or the second picture above.
If you make a single form and fabricate two sides from it, you could fur out the front to get the taper you need.
For the nose, why reinvent the wheel?
Proline Racks: Yakima SkyBox 16,18 and 21 Roof Boxes
These come in three sizes 36" wide by 15-18" tall (914.4x381-457.2mm). I have one of these that's bigger but shorter I bought cheap used. This style is intended for skis. It would need beefed up attachment points and (being thermoplastic) it would explode in a collision.
While I was looking for an illustration, I ran across this:
http://www.akmining.biz/snow/snowcoach_by_equinox.htm
Now this is made of polyethylene and it would be like riding in a big crash helmet.
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How much of a boat tail can you squeeze into the rules?
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Edit: I see your cross-post. You're putting wings on an open wheel car. If you could figure out how to make a panted, cycle-fendered wheel that created the downforce it would apply directly to the tire's contact patch. I don't know what that would look like.
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Boat tail.