Move the battery NOW. Keep the weight down and the plane will fly better. Extra weight is a last resort. Fishing weights or lead shot like used in reloading shotgun shells are both OK for the "Hole in the tail" method but I would rather see the battery hatch behind the wing. Weight removed from in front of the CG and added behind the CG is the best way to move the balance point with no weight gain. Can you move the engine back any?