I made a custom side scoop for the Fiero in August of 2003, here's how I did it and how it turned out.
First I got two extra stock intake panels, then cut the stock air intake out of one and cut a hole in the other under the stock intake.
I then fiberglassed the cut out scoop into the hole in the panel, and used some bondo death putty to smooth it out.
Did some sanding and then threw some primer on it
Did a little more snading and then painted the panel black and the grills the same silver as my ground effects.
The panel I did this on, as you can see in the above pic, came off a car that didn't have ground effects, so I had to drill a few holes for mounting the ground effects.
Took the original panel off the car and put the new one on :)