The problem's the timing, really; she seems to have done the thing up in a single night. I'd assume there were other Cass costumes lying around and she modified one, but it still seems implausible to me for a single person with no manufacturing tools to do modifications that extensive that still hold up to bat-abuse (just slicing down the seams of a kevlar vest and sewing more kevlar in the gap is not a great way to make a safe outfit) in that little time.