I decided to be the Women's Restroom Sign last year. I started with two big white cardboard pieces that I found at Target. They were in the school supplies aisle and were double fold out display boards, like you would use for a presentation.
I cut the two triangular front pieces and rectangular pieces for the sides. I got white thick elastic that I used for the straps, so it would hang over me like a sandwich board. I put all the pieces together with white duct tape and attached Velcro to the straps and inside the front piece so I could easily remove it. I also made one of the side panels so that it could open up by only taping it on one side and attaching Velcro to the other side so it opened up like a door. That way I didn't have to remove the whole thing over my head, I could slide out of it.
I got white sticker letters at Michael's and colored them bright blue with a Sharpie and stuck the word "Women" to the front. I also put it in braille underneath, by just cutting small circles out of some of the letters I didn't use.
For the head/ball, I got a large playground ball at Toys R Us - the largest size I could find. I initially spray painted it white. I popped it with a scissors and cut a hole big enough to fit my head through. Upon cutting it open, I found that the inside was white so I decided to turn it inside out and use the white inside on the outside. The ball material tears very easily when being turned inside out - I went through two trials. To prevent any further tearing on taking it on and off, I reinforced the edge of the hole with small pieces of white duct tape. I cut another hole large enough for my face. I put stuffing from the fabric store inside the ball around the sides and back so it would keep it's round shape.
Threw on some white tights, long sleeved white shirt, white mittens, and voila.