Actually I made this completely by hand, I used WorldEdit to create the sphere and than used spheres with a radius of 0 to change only one block at once. It took some hours and what you see in the final version was not the first attempt but it arose from many tries. The best way to create this ist start at the bottom and build up on more than one side at the same time, to make sure it fits with all sides, and finally connect them in the middle.