From experience I have had with an open chest costume, it kind of depends on how nice your boobs are naturally and how you want them to look in the costume.
If they are already nice like you want them without any extra support then you could use body tape to stick on the cups made of Foamies (aka craft foam).
If you want extra boob support then you should try to make a bra with clear straps, you could use/destroy a pre-made bra or make the cups yourself.
As people have mentioned, craft foam and paperclay (for the details) sound pretty good to make it out of. I've never worked with paperclay before so I'm not sure how strong or brittle it is.
I hope to see you at Blizzcon <3