It was made out of bondo and masonite primarily. The biggest struggle was removing the formed acrylic off the 'buck'. I tried a number of lubricants and coatings but always ended up prying on the thing and wrecking a few formed pieces in the process. After one frustrating day where I damaged the original, I began on a totally new vacu-forming rig. The custom buck was cast in a massive block of JB Weld with a wooden core and perforated in key locations to allow for good air flow & forming. The box was made of 3/4in. plywood and the buck sat on a threaded assembly that pulled the buck into the body of the vacu-forming to pop the formed polycarbonate loose. The new rig was an enormous improvement over the first one.
Buck dropping into the Vacu-Former:
The frame that I used to hold the acrylic and then polycarbonate in the oven has been the same since the beginning. It's a modified metal picture frame, even using the original clips to hold the plastic in position. After much experimenting, I devised a metal shield that guided the plastic as it began to soften and expand in the oven's heat.
A few weeks ago I was waiting in line at my local TAP Plastic store and thumbing through a book on Vacu-forming and it said something like "Polycarbonate is the most expensive and difficult plastic to Vacuform with the narrowest workable temperature range and prone to failure." and I thought to myself, NO SHIT. Even with a stop watch and digital thermometer Vacu-forming clear plastic is a big ole pain in the dick.
If you have questions about this process, feel free to ask. I probably know more about this junk than any regular person ought to.