If you look around the board, you'll find there are a set of templates
for armor too.
As far as cardboard + fiberglass...unless you are heart set on fiberglass armor, I'd just buy some of the pvc 'for sale' signs from the hardware store, or a garbage can or plastic containers, and do plastic armor. It'll be cheaper, and easier to work with. Once you cut your pieces from the plastic, you can heat them with a heat gun and shape them by hand. If you mess up, you can heat them again and reshape.
With fiberglass if you mess up, you'll either start again from scratch, or be sanding until you're blue in the face. For a first set of armor, I'd go plastic - much easier to learn on, cheaper and lighter. All the things you need to make it are at the hardware store too.