NOTE: I'm not going to go into as much step-by-step detail with some of these remaining pieces, so long as it's still made the same "clay model -> mold -> casting" way.
The chestplate was hard to make, although it turned out wonderfully. It took me a long time to get rolling on it, because I was having trouble working with the clay on this scale. The chestplate is so large, and made from 1/2" thick layers of clay, that it is extremely difficult to roll out sections of clay to use with a "rolling pin" (mine is actually pipe).
So I came up with the idea in the first picture. I transferred my pattern to a piece of MDF, and cut it out on the bandsaw. I then took the center cutout piece, and cut it down to remove the outer 1/2" or so all the way around. I screwed both pieces down to another piece of MDF, aligned like they are in the picture. I then heated the modeling clay in the microwave until it was very soft, and forced it into the makeshift "mold", and smoothed off the top. I then took the center piece out, and did it again. When those two clay things were combined, I have a 1" thick "armor plate", but the back is recessed with a 1/2" thick lip around it. These leaves the back "hollow", which will allow foam padding to be added.
You can see the results of the casting. I'm extremely pleased.
You're probably wondering what the last picture is. It's the beginnings of my friend's moonkin costume. It's a network of fiberglass poles, hooked together to loosely form the support "skeleton" for the moonkn. The foreground piece is the top, roughly from the armpits up. The background piece is the lower body, but it's upside down.