Tell us about yourself!
As shown on the 3d cad type drawing, I put a small round piece on the bottom and top of the guitar. That allowed me to bolt it down and to flip it over to do the back and keep it centered. The round pieces were cut off with a hack saw later and sanded smooth.
I have a Shopbot Standard 96"x48" machine. You can find them at shopbottools.com and see pictures there. In addition to using Rhino from rhino3d.com for modeling, my primary cnc software is Aspire from vectric.com. Aspire has modeling too, but I was not as proficient with it at the time I built my guitar.