Painting on latex (original model made from weight plate and clay).
Several layers of latex later and pretty much dried.
Fibreglass mother-mould -- not my neatest work! But it'll do; it only has to be strong enough for the latex to hold its shape.
Plaster poured in, it will ultimately be made from resin but for a cheaper test run plaster'll be fine and easy to smooth and shape if another mould has to be taken from it.
Demoulded, worked out ok! A couple of tweaks and the real deal will be ready to go.