cut existing cornice square, offer up a piece of card and draw round cornice
from drawn template construct a direct mould, run down on the bench with a backboard at 90 degrees to imitate the ceiling and wall angle. you can core out with damp sand to reduce weight.
(this is a easier than running insitu if you have no experience.
Square the cornice off using a multi tool, measure distances from wall and ceiling. Go to cornice site on interweb and you should be able to buy a 2.4 m length that matches what you have. Or give me a call im in your area and ill fill it with easi 20 filler