The Lower Torso
The Abdomen Plate
The abdomen plate is pretty much flat pieces of board glued together to make the necessary shape. When making a vacuumform plug you might want to use smooth laminated boards for this, or a dense MDF board. Metal plugs will work too, but more difficult to create.
The Abdomen plate has a slit cut in both sides of it to allow for the trench coat's waist belt to go through it. The slits are cut at the verticle middle of the abdomen plate. The first time I cut my slits I placed the slits towards the flatter side of the plate, and the plate never sat correct when I wore the costume.
There is a round metal disc in the center of the Abdomen plate, no one has yet been able to identify what this metal disc came from. The disc is seen on a number of other Star Wars costumes and equipment, so it was a fairly common item. The disc seems to have been metal stamped by a professional machine. Using a "laser caliper" device at the touring "Magic of the Myth" exhibit, we determined the outside of the disc is 4 cm in diameter, while the inner disc is 3 cm in diameter. The three smaller raised circles look to be 1 cm in diameter.
The Groin Plate
The groin plate contours to the human body so it is a very tall mold sitting on the vacuumtable. You can rivet a white web belt to the back of the groin plate and no one will know how you wear the groin plate with your costume because the trench coat will hide the belt.
Study the images from the Reference Pictures section of this website when sculpting your molds.