norton gave the answer that is the most informative and educational. A Baltic Birch plywood of several layers is the best...not osb which has to be able to swell and contract more than plywood - that's why it is good for wall and roof sheathing.
Read is used by what several custom O gauge engine turntable makers use.......it's either a many ply plywood ( 7 or 9 or more)...some flat out say baltic birch.
1/2 cdx plywood would be a good economical choice.