In aircraft pattern building there is a material that is called Phenolic. I am not sure of the exact and proper name for this material. It can be machined to very precise tolerances, is rigid and strong. You may want to check into that. This material would not be susceptible to dimensional instability in the way that wood could be possibly.

Additionally, carbon fiber would be a good material. This composite is finding it's way into many aircraft fuselage assemblies because it is strong and it is light.