I basically did that, except used a solid block of aircraft foam. Takes out a step. I used foam coated with fiberglass to make ribs underneath for strength and epoxied in rubber isolated bolt mounts to fasten it to the frame. It worked quite well and it only weighs about 4 lb with the seat in place. I need to take some pics underneath to show how its fastened, i know guys have asked before.
Bikes looking good though! You're having the seat cowl dipped for the carbon fiber look? Shoulda just used carbon instead of fiberglass, only a bit more expensive, and probably cheaper than the dip. But then you have to have your fibers oriented right to get that look without stretching the weave. Buddy of mine used to make carbon fiber skis ;D i got a lot of pointers from him.
Congrats on the first ride. I was there a couple weeks ago. Now i'm working the kinks out. Probably more frustrating than getting to the first ride :-X