I would suggest that at that speed you will be flying with quite a high reynolds number and a very low lift coefficient. A slope racer or F3B airfoil would do well on such a model. But then so would the Selig 3021 which is MUCH easier to build accuratley if you are doing a built up balsa wing. But take a hint from Dr Selig himself and extend the top sheeting back to the 60% chord mark to better support the covering through the tighter parts of the upper surface curve. The spars would still be at the max thickness point and the preffered structure would still be D tube but there would be some additional sheeting behind the top spar back to the 60% point. You'll have less drag that way and better slow speed performance for takeoffs and landings.