Tim's solution is certainly best. However, if the airplane is already built and finished, I usually just wrap solid core solder around the tail wheel strut and fly until I get the necessary weight then remove it and weigh it. Then carefully, with a single edge razor blade, slice out a rectangular "window', on the bottom, at about 40 degree angles on the sides, then epoxy the needed weight inside and replace the rectangle into the hole and glue in carefully with Sig-Bond aliphatic glue. The aliphatic glue doesn't affect the finish if carefully done and at worst it only needs a bit of touch up with paint!
Randy Cuberly