Wear nothing and be a rapist.
Or,
I sometimes just go to a costume store and look at the masks for inspiration for my costume. Last year I went as Steve from MC, all from Target. Year before I did a Bart Simpson. My best by far though, is a homemade stereotypical Mexican guy costume I did, all with things I had at home. It was complete with burlap satchel, authentic blanket and sombrero, toy cowboy revolver, and a mustache. I might post a picture, it was absolutely awesome.
EDIT: My friend went as a soldier, and wound up becoming the border patrol. Maybe you could get your friend to be a cop, and you could be a prisoner?