Making a body shell advice

[Mash Potato]

Hi all,

Trying to re-make a body that is no longer available. Have a shell to use to make a mould. Plan is to fill this with some sort of resin to make the mould to cast the rest of the shells from. Plaster of Paris has been mentioned but I have been advised this will not last long, hence looking at resins. Anyone have experience doing something similar and can offer advice on materials/resins to use?