I guess there are many ways to do it, I'd search for old threads and check the existing tutorials for body armor, some have good tips. I've heard some making a mock-up of their waist-to-neck and just build it on top of the mockup.. That way you get to build it specifically for your body shape. Another way I could think of (but would not recommend) is wearing 2-3 right t-shirts, and your friend putting wonderflex directly on yourself, and when it's cooled down, you'll have a breastplate that is a little bit roomy. 2-3 layers of wonderflex would definately be sturdy enough, if not, make it two layers and reinforce with another cheaper material.
If I made a breastplate I'd make it in two pieces, one front piece and one back piece and use velcro, straps or some kind of lacing to attach the two pieces together or to the body. I suppose it COULD be made in one piece but in general because of how a body is shaped, and especially the female body in this case, it would be easier to make it in two pieces. Make sure that it does not limit movements to much, atleast sitting down and walking should be possible, and moving your arms.
Armor will _ALWAYS_ restrict movement but can be made relatively well. Leaving room below the arm and not making it too far out on the shoulder is crucial. Making it too long (passing waist) would also limit movement a lot. Just remember to reinforce it so much that your body is partially stopped before it breaks or creases - else you will have a very hard time moving around in it.
Completed cosplays: Vincent Hitachiin Hikaru
Plans: Syaoran (casual), Nero & Kaito
Planned cons: Next Uppcon & Meuwcon