mold material for expoxied/fiber layup

[nurbs]

I have used freeman machiinable wax before, but its kinda costly for the application i'm currently working on for a mold material.

I need advice on a material that may be obtained in a 8 x 12 x 17inch so i may machine a mold.

so far i'm leaning towards a plastic or wax similiar to the freeman machinable wax.

but it needs to be able to release fairly well and not react with a system 2000 resin.
thanks