Boyer is on the right track, but I would add a
couple of things.

Generally the trailing edge lower 1/3rd of the foil is the offending part, and deserves attention first. Square
it off and then bevel one side at 45 degrees. His drawings
show about a 30 degree angle and thins the trailing
edge more than is needed. In fact, the thinner the trailing
edge, the more prone it is to flutter.

Take care of this, as vibrating the hell out of the
foil is bad bad bad for laminate structure
of the foil.