Rev,
Don't buy any of the commercially available NACA ducts that you see on NASCAR cars as they are really junk. Carol Smith has the info for building your own NACA duct in his "Engineer to Win" book and the important things are: Locate the duct in an area that has attached air flow, make the edges sharp, make a nice radius on the upper part of the inlet opening. Not hard to make and you can actually make them with some aluminum and epoxy if you can't weld alum. Or do as Stainless suggests find a "real" duct from an airplane and use it.
Rex