They do make a anti-cast surcingle... don't know how good it is or if it really works..
It is basically a surcingle with a metal bar attached at the top that prevents the horse from rolling completely over in the stall. Supposed to keep them from rolling and trapping their legs against the wall.
We actually used to use the things in winter to keep added blanket layers from sliding and twisting out of position.
I agree with other posters. Keep objects to a minimum in her stall she doesn't need and keep her stall bedded with shavings banked along the walls and in corners. It will take extra shavings to accomplish this, but they don't soil the banking normally.
A 12' x 12' box is plenty big enough for Sunni. She has room to lie down (she feels safe to do this)and is enjoying the soft bed of shavings.
Some horses are prone to casting themselves, some roll everyday, a few times a day and never cast.
Relax mom... Sunni will be fine. She seems to have adapted to living inside better than you.