This was done upside down on a party store small punching bag balanced on -a glass? (i think it was) . the trick for me was to support the item and watch all as it dries to the point i could turn it over. The balloon loses air and changes the balancing act as it is drying. A higher quality ballon or punching bag may hold the air better. but the clay is shrinking on it so exterting some force on the air nside the balloon. Hope this gives you ideas.
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