I'm
sure others much more expert than I will answer this, but you most certainly do
NOT want to fill the concave portion of the elevator. It is an important
part of the aerodynamic design, but I'm not sure of just what will happen if
the elevator section were to be straight. Also, it is important that the
round portion at the leading edge be flush or preferably "stand
proud" of the stabilizer surface. This is especially important of
the ailerons. I believe that is so that the boundary layer attaches itself
to the control surface. Most of the control, at least for small
deflections happens when the air follows that rounded surface, not by being
deflected by the trailing edge. Some builders have found this out the
hard way when the aileron forces are very high and roll control is less
responsive than it should be.