Currently I just have some great planes stick on weights on it. I plan on making some more changes that could effect the balance so I am holding off putting them in the wing. When I'm finally satisfied with the overall set up of the airplane I will probably remove the covering from the wingtip and make a slot to place the weights in and recover the wingtip. Nothing all that complicated to it really, just don't use glue that can attack foam and you'll be fine.