All of the 'custom' stuff I've buit I used the 'Lost Foam' method. I purchase the builders foam from Home Depot or Lowe's, use their glue to turn the 2" thick panels into somethin useful, then start carving away using airtools/80 grit sand paper. Once I get it where I want it (but just a bit too low), I put 3 coats of fiberglass over it, let that set up for a few days, then take a D/A and carve the foam out from behind the panels...then do the bodywork to make the panels look GOOD....
it's not superfast, but it sure beats building it in chicken wire, then claying over the top of that, then building a female mold off of THAT, then building a plug in your mold.....if you were going to build multiple peices, I could see all that work, but for a 1-off, it's unnecessary.
IMO
Check
THIS SITE. The guy builds a few custom carbon-fiber peices for a Suzuki (tank and tail...) using this method...and they came out pretty nice! Imagine building your own tanks!!
Phil