Before you start putting weights on your model, you may want to try moving your battery pack around to try to balance without adding weight. Most trainers with their long, heavy tails often require the battery to be placed up front with the fuel tank. If the plane is tail heavy with the battery up by the CG, you can take it out and use masking tape to position it on the fuselage while you check the CG. Once you determine where the battery pack needs to go, then you can mount it securely inside the fuselage. I have a few planes with an access hatch near the tail for the battery pack, this keeps the overall weight down and benefits your flight characteristics.

If I have to add weight, I usually drill a hole in the lead weight and fuselage, treat the fuselage hole with thin CA and then put the weight on with the two sided tape and a screw. Glow fuel has a tendency to creep under almost anything and loosen it over time, so the screw helps ensure the weight stays attached.