Here's a really simple costume that I'm pretty proud of. Mad-Eye Moody is a really tough guy whose main accessory is, you guessed it, a "mad eye."
I crocheted this eye with white, blue and black yarn, then I sewed a circle of felt to the back and used a black elastic band to go around the head.
All you really need for the rest of the costume is a trench coat and a pair of combat boots. (Messy hair is also a plus)