These are really easy to make...

Take the NACA drawings, (link posted above by me) and scale them to the size you need, then make a plug, against a flat surface, so you have a glueing flange when you're done. Finish it with glass, or something that will stand up to molding, and you're done.

I first made a female mold, then realized that a male mold would be better, and produce a more structurally sound part. That part of my jet article is at http://www.nextcraft.com/mold_naca_inlet.html

The male "mold" allows the "shiny" or smooth side of the molded part to be the one on the outside, and it also allows you to make a small fillet along the sides, which would be inside the plane...not possible with a female mold. For your reference: To produce 3 sq. in. of intake area, the NACA inlet will have to be about 8 inches long, and about 3 inches tall. (I'm using two, one on each side of the rear fuselage, on my jet.)