O.O Well, this one should be interesting!
One suggestion -- Use a wire frame to keep the bowl round? And maybe have a lining of 2 inch foam inside to give it that solid look... I have seen a costume before, but I think the bowl was made (cut off about the thighs with leg holes/elastic) with a wire hoop to keep that round idea, and then held up with suspenders... Maybe if you use the same idea, you could paint the straps to match the rest of you and they wouldn't be as noticable? I might not make any sense.... But post pictures when you finish.