If it weren't for safety, I'd make the lower legs solid and stand as if walking on stilts about where the knees would be. It would be a humongous costume due to scale and you'd probably break your legs trying to walk around in it (not to mention the fact that you would need to see somehow), but the proportions would be right. There's just not enough room for an entire torso in that ball if the legs are the size of normal human legs.
I'd love to see this attempted, though.

all you need is a old sea mine emptie it make a plastic form