I broke the 1/2 the mold for the gear pictured, but have since been working on a mold exactly like scole has described. I used MDF in place of the particle board. It's a little harder, more like compacted dust, but really anything will work. Get gloss Formica, and get some nice uniform coats of PVA on the mating surfaces. I have switched my process a little. I now put a thin layer of epoxy on the surface, and wait for it to gel up, then make my layup on a piece of glass. Epoxy, CF, squeegee, epoxy, CF, squeegee, epoxy, CF, squeegee, ..... Let it sit for a bit, until it's almost at the tack stage, then put it in the mold, compress and bake. If you can catch it in time, it gets to an almost hard phase, that allows for easy trimming. It's really got to be an experience 1st hand kinda thing. You can practice on cheap glass a couple of times, to get a feel for it.
Try it, it's fun to do. I've been making CF props lately. I'm working on a big one right now, 22x8. CF parts look so cool when they're done.
--Rick