Our Mouldphorm PCL Low-Melt Thermoplastic is a good temporary mould material.  Dead easy to use and epoxy, PU,  polyester and vinylester don't stick to it so no need for release agents.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

Have a look at polymorph I have used this in the past , you need to take into account the shrinkage as it cools down.

Or maybe wood?

Has anybody tried making a mould out of nylon or some other kind of easily machinable material? I am going to make an aluminum mould for the final part but would like to try out some kind of quick mould first before investing the time in machining the aluminum. I know the surface finish won't be as good though. Thanks