I drill a hole and poor in epoxy and lead shot. This is very heavy stuff and it doesn't take up much room. Balsa block wing tips are great for this. I balance the plane with a little shot sitting in a pill container. When equilibrium is established, weight the lead and container to determine their combined weight. Reduce this by 1/4 of an ounce to compensate for the weight of the epoxy. Drill, poor, recover. The bottom of the tips is best and virtually unnoticed. If lead is needed in the tail of the airplane , I generally add it before final assembly.