Looking at pictures of the EF Extra I see that there are aerodynamic balance tab on the elevators. That's the part at the tips ahead of the hinge line. The actual fixed stab looks fairly small and likely pretty stiff... I would hope at least.... And often flutter occurs in the movable control surfaces. With the forward tabs you have an opportunity to alter the resonant frequency by adding some weight along the leading edge of that balance horn to shift the CG of the elevator towards the hinge line. If you needed some tail weight to trim the model anyway you could just move a little of it to the elevator horns. Adding the weight could be easily done by drilling through just a little behind the leading edge from the tip or the inside rib of the balance tab. Then stick in probably 1oz of lead round bar. You can buy the lead in 1/4" round from fishing supply outlets. Or flatten it and install it as a flat bar.

Ideally it would be enough that the elevators when not hooked to the servo balance on the hinges. But if you can at least move it some significant amount closer then that would be enough to damp out and avoid the resonance that caused the flutter.

Hope that helps.