Do you absolutely have to carve it? I would think if it was small enough to go on top of an 8" cake, then it would be much easier to make it out of RKT. If you just HAVE to carve one the best advice I can give you is to get a Nerf ball and make constant comparisons to the size and shape while you do it.
how small? i dont know if i could be bothered carving anything too small, id get a polystyrene ball and cover that! or just a ball of fondant.
if you want it in cake, you could start with rounds, but for that small probably easier to think about starting with a cube of cake, take off the corners and start working on making it ball shaped. sharp knife!
xx
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