Very cool, Toni!
I hope this isn't a thread-jack, but I'm reminded of a similar but larger-scale solution that I saw back in 1980 as a freshman at BYU. One of my classes did a "field trip" ... all the way to a building across the street from the football stadium.
The sign over the door said "The Ossuary"; this is where they prepared dinosaur skeletons for display.
Well, some professor had discovered a huge dinosaur (Ultrasaurus or Supersaurus, I don't remember which). Unfortunately, they had only found one of the two huge leg bones (the femurs, I guess) ... the other one was missing.
Hmmm ... how to make a reverse copy of the bone? You can't make a cast; that method will give you an exact copy, not a reverse one.
In the end, they set up two large tables separated by a couple feet of space. On the one table they put the original femur bone, and on the other table they put a big chunk of modeling clay. In between the tables, they placed a large (6-to-7-foot-tall) tool that looked like a drill press, but instead of a single motorized headstock it had two passive heads. The heads were at the ends of two arms that were geared to swing in opposite directions; if you moved one arm to the left, the other would swing the same distance to the right. The whole assembly could be cranked up and down, and moved in parallel with the tables.
In use, the technicians fixed two rods (of matching lengths) into the heads and then "traced" the actual bone with one of the rods. Wherever the opposing rod encountered the clay, they would carve the clay away until it matched. Fairly low-tech, and it took a long time, but it allowed them to complete the skeleton.