Anthony,
I'm not sure about the way I'm about to suggest as I have never tried it myself, but I suspect that most Aftermarket vendors do it like that. I'd suggest making a (wooden?) master of the intake itself (that is, the hollow part through which the air passes to the engine) and then "coat" it with sheet styrene, then cutting it in half and regluing together, plus any additional putty work. I know it has the drawback of a seam, but in the end, most default kit intakes suffer from the same "fault".
If you're building the 1/48 Arrow (minicraft I suppose), you can also check Mastercasters' intake faces.
HTH