Size could be an issue. Silicon tend to be flimsy and the weight of a 12” cake could cause the batter to over flow the sides.
That is why I want it to be several molds that can be baked separately and then layered together later to form a smooth sphere when covered with icing. For a 12 inch diameter I would need the bottom to be 2.51 inches in depth. The middle and upper sections of the sphere would need to be an additional 3.49 inches total (2 sections) to make it a radius total of 6.00 inches. The bottom 2.51 inches would be used again to make the base of the sphere. If anybody needs further explanation I can provide a drawing and verbal explanation on video.