I've been reasonably successful with building the mould for my front bodywork (which has gone very well for a first attempt, picture is as it came off the bodywork).

I have additive for the gel coat I used on the moulds for making repairs, which is why I was thinking I could fill it with gel coat, similar to described with the plasticine/ wax above to effectively fill it up on one side while using release agent on the mating part to ensure they will still separate. This, or similar using wax/ plasticine filler but with a skim of gel coat over the top, and ensure it gets plenty release agent before building the part.