Size the coax for the particular impedance you desire. Take a look at the dimensions of the commercial cables you might use in fabrication as a starting point. Generally the airbox should have about a quarter wavelength of clearance around your model, more if you have a strong computer with plenty of RAM.
Your original question was in August of last year.
Have you been working on this problem since then? To make a meaningful response I need more details of your situation. Just how you intend to use an airbox with the coax is a mystery.