Why not a simple lightly eased metal mixing bowL? (Perhaps I’ve basically misunderstood the problem...) They’re cheap, and can be bought in stacked sets for varous diameters.)
Again, maybe I’ve missed the key point...
“eased” shouldda been “greased”
Since this would have to be baked in sections, I would need silicone. The molds would make up with three sections of the upper sphere to it's widest diameter (12") with the bottom 2.51" used again to make the lower section of the sphere as the base. So it wouldn't be a hemisphere (or half sphere) but a hemisphere PLUS a portion of 2.51" additional sphere.