Curious to see how it looks inside my props? A medley of cardboard and reinforcement galore~ Most of this will be hidden once I add the corrugated cardboard, so might as well show this now. The left is the back breastplate, while the random paper cutouts on the right are for the front. I made around 3 patterns, until I finally found something I liked.
Cereal boxes, card stock and poster board are all very useful materials, but flimsy if built in large pieces like this. You want to try to avoid keeping it hollow. Using strips inside, allows me to connect two pieces together cleanly. It also strengthened my armour, to the point where I can put moderate pressure on the surface, and it'll support my weight. (Example, think encyclopedia size book, not damaging it, or crushing it. This is still while it's hollow, it'll only get stronger when you add more cardboard.)
As for pattern drafting, don't be afraid to use paper, and some tape to see if it works out, before using your good material. Once you get a fairly good shape, it's a lot easier to add or subtract amounts until it's perfect. :)