I was looking for a Death Star cake as I will be doing one next year. I found this thread. I have done ball cakes before and they are not that difficult. The one that I will be doing will be made in a 10" stainless steel bowl. I will bake two cakes and one 10" round 1" deep cake. I will attach the "bowl" cake to foamcore board and support with skewers. Then place a 10" round foamcore board on the top. The 1" cake will go there with a little buttercream on the top. The other bowl cake will go on top with skewers going down to the foam core. Ice and smooth in buttercream. Roll out fondant and drop over. For a 10" round sphere about 38" round of fondant so that the weight will pull it to the bottom of the cake as you are smoothing it.
My sphere cakes have came out perfect. A friend recently did a 14" golf ball but the bottom half was styrofoam made specifically for her at Dallas Foam company.