Is the aileron full span with the outboard edge of the aileron 5 inches from the tip or are the ailerons 5 inches long with the inboard edge the cut shown and go to the tip? It makes a difference in the answer.

If the ailerons are torque rod driven the stiffness of the torque rod is extremely important. It might be hard to get the wire big enough to not flex.

The other suggestions are great, stiffness of the system and mass balance are the two of the main things to reduce flutter. I would tend to blame lack of system stiffness first.