In plaster I recall my now late grandfather making the zinc templates to match existing cornice by painting the existing one with olive oil (no idea if extra virgin or not
) and applying Plaster of Paris as a mass. Once set, remove and saw through the middle of the plaster profile to give a flat piece with the profile, then scribe round it onto zinc and cut with snips.
this was what got attached to any formers etc to run the new cornice. IIRC it was roughed out with plaster and scrim, then a finer paste used with the profile for finish.
All subject to the memory of an 8 year old at his grandads knee, but might be useful info.