A similar yet totally different "casting-type solution" involves spray foam. Some of which, sometimes dissolves some styrofoams.Originally Posted by winchman
Another solution, is melting the half moon into the big styrofoam ball by lowering a very hot metal ball into the hemisphere of foam (must lower it, or air currents will make strange varying thickness problems). A sacrificial stainless steel kitchen mixing bowl with a torch waved around the inside of it, might work, maybe. If you were feeling really reckless, you could fill the stainless steel mixing bowl with liquid lead... although I'd not use it to mix cakes after that.
Or last but not least, your primary problem seems to be no one makes a curved hot knife... Although you probably could make one... Something like a hot air heat gun blowing into a copper tube bent half round, then lower that thing into a rotating hemisphere? Probably an excellent way to burn out a heat gun, but a fun way to do it.