So, my plan is to build up the base of the underside to match what is currently included on the lower half of the original. This will simply be a platform on which to place the greeblies. I'm using styrene because It's easy to work with, and will provide a predictably flat surface when all is said and done.
I placed the grey car bottom side down on a piece of .07 inch styrene. I used that thickness because it was the thickest I had on hand. I expect to do a couple of layers of styrene, after which I will sand in the bevel that is seen on the master. I traced the outline of the car shape on the styrene using my handy Alberta Industrial Models pen, leaving a couple millimeters lip, in case I screw up. I used my coping saw to cut out the shape, then sanded it a bit on the back end to get the proper arch shape I want.
Before attaching the styrene, I made sure the bottom of the car was as level as I could get it. I had to dremel out that big blob of bubbles that you see towards the front of the chassis in the above photos. The bubbles created a ridge in the underside that would have given a contour to the styrene.
I ran out of epoxy, so I used a household adhesive that I got from Home Depot. I think it's called "goo". Before applying it, I tested it on some styrene. It held like a mofo. It is an air dry adhesive, which is not my favorite, but oh well. I applied the goo to the underside of the car, then spread it thin with a popsickle stick. I then placed the styrene on the car, and pressed down on the top of the car with my hand to distribute the weight, and to squeeze out excess glue. It worked pretty well, and I now have a grey car with a very flat underside.
While waiting for the glue to dry, I took another look at the grey undercarriage to see if there were any parts worth salvaging. Take a look at the third photo of the grey car in "Step One" of this process. See the weird bulbous thing in between the two long engines at the back of the car? That piece was originally lying loose. It is not attached to the undercarriage, but is positioned correctly in the photo. I took a closer look at that piece, and figured it for scratch built. For that reason, I though it was worth salvaging. I sanded the underside smooth, and razor bladed off a bunch of "boogers" that were around the bottom. it turned out nicely, and I plan on incorporating it directly into the final pattern. I expect to do the same with the ridge shape that is towards the front of the undercarriage. Although a friend of mine guessed that it might be the front cowling of the Return of the Jedi Speeder Bike model kit, I beleive the size is wrong. It does indeed look similar, but the uneven shape suggests that it's a scratch build piece. As I get the underside better mapped out, I will extract that shape and clean it up.