I do it like RiverMan explained. However sometimes with balsa I don't even Dremel out a channel, I just lay the wire in and then glue the two halves together using a vise with a little pressure. The balsa will conform to the wire and you don't even have to worry about the channel. Also you usually want the weight as low as possible so it usually doesn't bother the through wire. If so just do as others suggested, run it a little higher in the body. I've never found that it throws the weight off.
Good luck