Ok, back to the first post, how to get best results without help of composites.
Two kind of unwanted deformation has been mentioned; deformation under stress, meaning torsional stifness and deformation during test of time, meaning warping.
The dilemma with balsa(or any)wood is that regardless of density, the individual wood cells are allways more or less the same. This means that in denser, heavier, wood there is more wood cells per volume unit.
Because of that, denser wood is more prone to warping, so that's the price you pay for using heavier wood to increase stifness.
And the other way, lighter wood is more stable but with lower stifness.
So the best way would be to use light, quarter grain cut wood and to keep thickness as thick as possible.
Reinforcing the trailing edge with something harder, carbon or hardwood, is nice especially against handling damage but as it is located at the central axis of flap, it won't do much to increase stifness.
If you really want to do it right, the best way is to use skin with modulus of elasticity higher than balsa's, kept solidly apart from each others with a lightweight core. The core can be dense ribs or some lightweight foam, like Rohacell, or if you want to go a step further, honeycomb.
The skin could be of thin balsa/glassfiber laminate, thin plywood or thin carbon sheet. There are really nice thin carbon products easily available, starting from under 50g/m2 for a nice biaxial product. Please don't confuse them with carbon veil, the veil has nothing to do with correct use of carbon fiber. L