The most simple are the "drop boxes" that dangle at the hips. These are simple rectuangular molds, with risers. You really only have to make one. Details might include a slight taper, and a slightly rounder corners.

Don't forget that these molds must rise above your vacuum form machine's forming platen [forming surface] and that a 1/2 inch or so will be necessary to raise the mold high enough to avoid any soft edging. The bown area below the mold in the upper image is the riser.

If you can make the drop box, you can make the next mold, the knee power-pack.

Note, the image shows the collar strap too, but I had decided to rework this mold. Also note, the knee power pack needs some metal "rods" to be glued in place in between the raised bozes. An old thin wire coat hanger, or even some 14-2-G electrical wire will do. This will allow this part to bend around the knee. Otherwise it will be very rigid, like the collar straps.

Next, lets continue by making the hand armor: