Well then... here's a side view of my cod piece.
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All you really have to do, is make the flat cod piece then curve it to your body. Once that's done, make a thin cardboard ( cereal box card works great ) for the front curve ( the part that curves down to, well... you know ). You'll want to make the bottom part of the curve first... I simply drew a nice flowing curve and folded the card in half to cut it out. ( this will ensure that both sides are the same )
Once you're happy with the shape and placement of the bottom curve, then simply trim off the top portion where it will attach to the cod piece. This cardboard will now serve as your tracing template so that you can make it out of sintra/plastic/metal or what ever you're making your armor out of. 
Hope that helps. :cheers