Put on a large t-shirt that covers your bottom. Have a friend wrap duct tape around your body starting at the bottom of the t-shirt. You will probably need two rolls of duct tape. At the bust area you will need to do some special taping (see tutorial).
When the entire torso has been covered cut off the tape & t-shirt in a vertical line up the middle of the back. Remove the form and apply a layer of tape to the cut portion. Cover the arm hole and bottom opening and stuff.