Here is how to add ballast in a north star.

Figure out how much lead shot you need in the nose to make the plane balances slightly tail heavy on the cg (the epoxy will make up for the rest), a bored out nose block helps, place the plane on its nose and secure it to a chair or ladder, pour the lead in the nose area and then pour low viscosity epoxy over it. This has worked great on the North Star, Arrow and a Sea plane I designed

SGG