The taper used in the elevons reduces their mass near the tips, where their mass is most likely to induce flutter because simple wing flexing vibratory excursion is highest near the tips.

Control surfaces should be as light as practical while retaining sufficient strength to do the job. What they need in order to minimize the likelyhood of flutter is high stiffness to weight ratio, not maximum possible stiffness at the expense of weight. I would be strongly inclined to employ mass balancers at the tips of the elevons. I employ them on all of my airplanes that are expected to fly fast enough to make flutter a possibility. The mass balancers, if mounted at the tips, should statically balance about a third of the static unbalance of the elevons.

Excessively heavy mass balancers can be more dangerous than none, since they may induce flutter a flutter mode where the wing or stab flexes in a more complex manner than simple flexing of the tips up and down.

Such higher harmonics usually only appear at rather high airspeed, where flutter can be extremely brutal, and break up the airplane in a fraction of a second.