If you have an Air Brush of any type you can do most of what I did here.
First off I put a very thin wash in the air brush. Sorry I have no idea what ratio, I mixed what I used for my Camel Porsche years ago. But when doing this stuff you want to build layers anyway.
Set the car with the front end raised about an 1"
Shoot the air brush at the front end varying the angles, up down, side to side etc etc, simulate air movment over the car. You can simulate air speed variances with different air volumes and psi.
I do this a number of times, building up layers.
Then I go around with a very low psi and narrow fan playing with what my minds eye sees. I am curious to see what 1:1 NACA ducts actually look like. I can't imagine they have that much of a tarnished intake flow signature.
I hope this helps you some what, good luck man.