Plus, it depends which costume you're planning on using. Mystique had one main costume, the white dress: Click
but she also, in Evolution, had this one: Click
. The white dress would be the most accurate, as it's what she generally wears in canon, but it might be difficult unless you either a) have a dress like that or b) can manufacture one. To do this, you could go to a charity shop and buy a long white dress and then edit it to make it like Mystique's own. The skull belt might be difficult, but again it can be done with a bit of work. Get a belt and make the skulls from cardboard and glue or tie them on.