Aloha Paso!

NURBS are great! Probably what you may want to try there is to:
Seperate your top profile curve up a bit.
Then extrude your base curve up higher than the desired thickness of the guitar body.
Then sweep your profile curve (Guitar face) into a surface. (You could make a better face for this than you show to produce a better surface with this method.)

Then place that surface at the desired guitar body depth and boolen diff to create the shape you want.

Good luck.