In the old days, builders made bulkheads and tacked stringers on to make body plugs.
Steve Graber built a giant gantry router to make the plug for his LaBala design, and CNC milled the plug out of foam.
There have been a few 3D printed bodies. But among the printing technologies there's a "road less traveled" that I keep going back and looking at:
http://candyfab.org/"The future will be carmelized." Yes, they're printing ordinary sugar, like you buy at Safeway. The printing resolution is low, but it can be covered with plaster and sanded. And if you have any programming oopsies, you can eat your mistakes...