Drag Coefficient of Box Lights Atop a Police Cruiser?

I hope someone here can tell me how much horsepower is used up by the
lightbox on the roof of patrol cars at 60 mph. Assuming the box is 48"
long by 8" high, what is the pressure of the air against it at that
speed? How much hp does it take to provide that amount of pressure?
And if a car gets 10 miles per gallon, how much more gas is used up by
the lightbox added on?

I have tried several ways to make those determinations, and I can get
the rate of flow, but not the amount of drag or the amount of hp needed
to push it. TIA.

.