Oh. Depends on how do you wish to use the pieces you make. I assume you're making foam pieces for your costume? Will it be used over your skin or over clothes? If it's over skin, I'd try keeping the pieces held onto the dummy without damaging it. Maybe you can hold them up with doudle faced tape? Just to keep them in place and fitted well, in a way that you can later take them off, after finishing, without getting your dummy damaged.