[Ant]

You could use plywood, but it will need duct tape or release agent attached to it, boat builders etc use aluminium sheet, so if you have some alloy sheeting or plastic just whatever to form your mould, as mentioned by Dwight duct tape, alloy sheet or plastic doesn't need a parting agent at all, they should just lever carefully off. Using a smooth surfaces like these leaves a nice job that will need less preparation, and the non mould side make sure you keep that with the brushed resin/matt side, or identify which side needs to be a nice finish, and what doesnt in placing your mould backing.

If you can make your mould so when you move it a small distance it clears all the glasswork, (there is a name for this principle) as in big jobs the mould must clear the job and not compress or damage the job as it comes off, so the moulds largest area should be at the end further in on the job.