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The internal surface shouldn't really matter at all, since what you're aiming for is just a big volume of air, rather than a smooth surface for the air to flow over. The surface would only matter if the airbox was acting as a restrictor, so that you're trying to squeeze as much air through a confined space as possible. If you airbox is acting like a restrictor, you be doing it wrong. In any event, a little bit of turbulence is not necessarily a bad thing. Cylinder head ports flow better with a slightly rough surface than a polished one.
Brommers
Thrustvector
The internal surface matters more, since I'd hope he'll spend more time driving it than looking under the bonnet.