Okay, I'm kinda going by memory on her outfit, but...
Perhaps start with a metallic-y/pvc white or silver corset (they do make those, check ebay) and build up the armor shape with something lightweight and plastic that you can easily spray-paint and then attach (either with straps or hot glue.) Check used sporting goods for various parts of protective gear or even plastic serving trays or plastic containers that are close to the right shape. I'd advise against trying to cut/sand plexiglass (it's really hard and noisy to do) or trying to use any kind of clay (which won't be smooth enough, but would be heavy.) If you think you could sand it down and then fill the texture with several gloss layers, plaster gauze -might- work.
Another thing you might want to try is corrugated plastic boards. Very lightweight and very sturdy. And easily cuttable. I'm making a guitar out that stuff right now. Build up a couple layers, using stairsteps to form the curved angles, and then fill in those sharp jagged edges with putty before you paint. It might be a lot of work to do that, tho, in your case, because I don't think her armor is that complicated.
Also, if you are good with fabric, never under-estimate the power of the flatfold vinyl, which can be sewn.