Actually, if your fabric is stiff enough, the fact that it comes in and meets the legs will be sufficient to keep it puffy.
I would probably develop the ball shape, but complete it by sewing the leg openings and waist band to a pair of dance trunks. that will make it more comfortable to wear.
Last, the human body is not symmetrical front & back, the leg openings will need to be skewed to the front to allow you to sit down & walk comfortably. It will feel like less puff in back when it appears the same amount of puff front & back. (unless you have no butt at all)