when I did my mold from plaster I pulled the cowl front out of the plaster after it was set, about a hour, so it would release from the plaster cast, plaster tends to shrink,just a littlewhen drying so it can tighten upon a subject, it took a few days to fully cure after that.
for a release agentI used pure carnuba car wax, the one I used was a liquidtype wax, but paste wax in the can works as well, after at least 3 coats buffing each before applying the next then the molding process can be done.
that being said getting the cowl out of the mold depends on the type of mold being made, is it a split mold(2 halfs) or a one piece which is tough to get a part out of, if you have a compressor then a air nozzlehelps get the plastic part off the walls of the mold so you can get it out .
in the link of my build I posted in your Q&AI go into detail on how I made my mold from fiberglass but the same concepts can be used for plaster as well, after layingup a liquid base over thesubject plaster wrap(the kind for armcasts)can be used to make a thin strong layer so a large mold isn't needed.

photos:
my wax
subject in plaster mold
cured mold sealed with oil based Glosspolyurathane
finished cowl front