Pacman costume help please lol?

Anyway me and my mates are just gonna mess about and i am dressing up as pacman and they are the ghost's, i no this has been done loads of time before but i don't care, anyway i want to make a sphere one but if not i will make a flat one lol. if you know how to make a sphere one then please share but if not, how big of a box do i need to make a flat one? thanks