[Cd]

I plan to add a sign to the top of my car, but would like to know the possible aerodynamic drawbacks.
I plan to create a sign with as minimal frontal area as possible when seen from the front of the car.
( The sign will be no more than 1/4 " thick, and preferably much thinner.)
I'm concerned about vortex shedding, and would like advice on the ideal shape to reduce it.

Of course, I'm going to take a big hit when it comes to crosswind drag, but this is unavoidable.
Any ideas on making the crosswind drag less of a problem ?
( What about using fine mesh window screen and painting my logo onto that ? Nah...not visible enough.)

What I have in mind is sort of a huge mohawk 'if you will' ( did i use that saying right ? ), or a sort of 'spine extension' on top of the roof.

Lastly, what would you all recommend as a streamlined way to attach it to the car ?
( Simple, large 'L' brackets ? )

I have no worries about drilling into the car, or otherwise 'destroying' the cars looks .....it's a junker business car.
I only worry about it blowing off, and leaks - but think the leaks should be fine if I caulk up the areas where the holes would be.

I think my main frontal drag will come from any brackets and such that I use to keep the sign on the car.

I'll post some design sketches later, and do some sloppy CFD tests.
I'll let you all know what kind of drag hit I'm looking at.