I would advise against epoxy/lead or any method that does not allow changing the balance with low effort.

Here is the why.

The basic trainer is designed for self-recovery in pitch and roll. This means the the airplane is very speed sensitive in pitch. This is done by having the airplane balance very nose heavy, which in combination with the decalage of the wing and tail plus engine thrust line results in a model that flys level at one speed. Slight speed increases results in a climb and slowing down (lijke in a prolonged climb) cause the pitch to decrease automaticly. All these features are good if you have limited experience flying.

However as a pilot's skills improve, they soon tire of the self-recovery aspects of the basic trainer. So you move the CG back, and after you retrim, much of the self recovery goes away. It really doesn't matter too much what airfoil the model is designed with, though I prefer a good semi-symetrical airfoil for trainers as it allows more advanced flight than the basic flat bottom airfoil. You may also want to experiment with removing some of the positive incidence that trainers often have as well as the down thrust as the CG is moved aft.

By understanding some of the aerodynamic forces involved and changing a few things, your trainer can advance your skills far more than most would believe.