Wow! To hold the weight of a person, the hub will need to be extremely heavy duty. I would search the local auto wreckers for a used wheel hub. I'd then use the rim of a spare tire to mount to the backside of an actual large wheel.
I made an oversized "prize wheel" last year, and with mine one of the key issues was balance. Balance had to be spot on, especially with a bigger and heavier wheel. You could search craigslist & Goodwill for a large round wooden dining room table. This would have a finished edge and might split into 2 parts for handling. Sounds like a bold venture- good luck and take some pictures if you pull it off.
And PS, don't forget a pickle fork and big hammer if you go scrounging at the junkyard!