My brother Nathan, who works for UPS suggested those omnidirectional rollers, and I was going to try and use them, but the rollers I am getting are ready to go:
(click on image for link)
I've ordered four of them, which should do the trick.
I will be making an alignment jig, once I figure out how to set up the USB CNC program. I am leaving 1.5" at each short edge and .375" on each long edge -- the friction rollers need 1.25" minimum to keep a "grip" on the sheet and there is a .25" tongue/groove on the long edges.
Jigsaw puzzle joint makes sense to me. The slowest part of this process is that I have to "sum" the two adjacent sections -- they are cut starting at the high point of the roof, and the ones I am working on are cut "looking" forward of this. I then cut the next section, and layout the inside profile -- and then I copy the entire second section over the first one. I then erase all the lines on the inside; keeping the outer-most lines only. This makes the stepped form will always be larger than the finished shape, and I will only have to *remove* the steps to get the actual form of the car.
This was the major lesson of making the quarter scale model -- I had lots of voids wherever the shape "grew" from the previous section. And on the full size one I am now doing, it is the *inside* surfaces that need to be taken into account!