Hi there from down under If you want to do it on the cheap you could use latex rubber from dental supplies . powder or liquid, and bondo, use only a small ammount of hardner in the bondo to give more working time, if the mould does'nt have to be flexible you can use bondo also for this.....I thin the bondo down to thick runny, with resin and use it as a gel coat on the mould first, then back it up with regular bondo. I use plasticene or as you yanks might call it modelling clay ( the stuff that is oil based) to give a half mold centre line. If you want the mold to last ,back the bondo up with plaster to stop it distorting with time....a coat of pva glue brushed onto the bondo before the plaster helps the plaster stick. dont forget the pva release agent first!