This costume was originally made for cosplay at NYCC this year, but obviously can be used as a Halloween costume. I got the idea from a WFW video. This is a great last minute costume; Its really easy and cheap and only takes only a tee shirt, craft foam, belt, and shorts.
Step 1: Materials
-White/light colored pair of shorts or pants
-Thick black craft foam board
-White craft foam board, preferentially with an adhesive side
-Glue (If you don't have adhesive craft foam)
Step 2: Making the Button
Edit: After a day at Comic Con, I realized the adhesive backing on the ends of the strip by themselves don't hold very well. It might be a good idea to attach the strips to the back of the black circle first, staple them, then attach the white adhesive circle to the front to hide the staples.