What a hassle! You failed to mention which aileron is fluttering. Some of the solutions would probably work, and others would likely compound your problem.

First off, why is this bird using strip ailerons? The original design, of the Mustang, was to have slightly smaller than barn-door style ailerons, covering 60% of the outer trailing edge, and flaps utilizing the remaining trailing edge. The break in the leading edge, close to the root of the wing, creates a minor turbulence just at the root, and towards the trailing edge. It effects about 1/3rd of the supposed flap area, which is a problem onlyif the ailerons run all the way to the root. The right side less affected than the left, usually.

If it were my plane, depending on the manufacturer, I would install flaps and originally intended ailerons.

OR, if you are using a double servo setup, I have had success in moving the aileron control horn aft, just a scoche, behing the hinge line.

Don't give up on the bird, whatever you do! Trial and error, that will get you through this.

Keep your head up on a swivel,

Skratchbilt