This Naca duct will fit in the headlight lid of an NA miata with no modifications to the headlight assembly. Cutting and modifications of the lid itself will be necessary.
To mount, make the centerline of the duct 3 3/4" and parallel with the inside edge of the headlight lid. Then move the duct as far toward the back of the car as possible. This alignment ensures clearance of the headlight assembly.
Send me a message if you have any questions about mounting or finishing or see http://www.tommyhaake.com/current/#/naca/
for more info.