This is a more interesting challence - a piece of cornice had been removed from a room leaving just a poorly troweled run of poly-filler about 1 meter long. We managed to restore it.
You do this by cutting a sheet of zinc to the same profile as the existing plaster. Then fix this to a wooden slide. Fix up some rails on the wall and ceiling for the slide to run along. Then slowly build up the plaster work by putting plaster of paris onto the wall, and running the wooden slide and its template along the runners to take off plaster that is surplus.
Eventually we end up with the restored profile.