New Project with complicated mold

Ok now I know this isnt REALLY an interior question, its actually exterior/engine, but this is where all of the knowledge of fiberglassing and mold making is.  Ok Im making a fiberglass piece for my friends 2001 Mazda Protege MP3, he was going to buy a piece online that is like an air duct to route cold air into his cold air intake (novel idea right?).   Well he was gonna pay big bucks for it and I offered to try to make one, it will go from the right side of his engine bay, where the intake is routed, and channel through under the car in front of the left front wheel.  There is a little black lip type of thing that actually attaches to part of the wheel well and I will end up either removing or cutting through that in order to do this project.  Its really hard to explain but I will try to take pictures and post them.

Anyway my real question is, the mold needs to be made out of some sort of foam, Ive checked the forums and I see a few options, Im on a budget since this isnt even my car this is going on.  I saw the wonderfoam and the 2 part foam, I was wondering if there is a local way to find anything like this or what would you recommend to use for it, I know there are tons of different ways. 

The mold will also be hard to do, because I dont know if I have to make a plug, then make a mold off of that and then make the fiberglass piece this way, Ive never done any mold make and wonder how I should go about it, I need to make it so it will fit nice and snug in the opening but still need it to have a channel in the middle for air to come through.  Thanks for any help, Ill post pics as soon as I can.

my friend (who has a neon) told me about this a while a go. http://www.neons.org/forums/viewtopic.php?t=173873&highlight=intake+foam

Its a pretty good concept and i almost did it for a CAI

how are you planning to get the shape? is it working? you could try pvc pipe for some of it. i know thats ghetto, but it will save you some time and a LOT of work. and you could even glass over it and paint it like everything else... good luck man!