Check out the tutorial here on making witch cauldron. My wife and I made one last year using cardboard spars abound a plastic tub. With a little modification you can make a ball using this method. We doubled the amount of spars to lessen the facet effect and then I rounded it out with paper mache clay as you did. By using the extra spars it required less clay therefore less weight. It was still a lot of work, a lot of drying time and it got heavy. But it did work. When finished I gave it three coats of truck bed liner for protection. Here is pic of it so that you can see the roundness we achieved.
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