I've always used a left over 4-40 rod. I bend it in a U-shape, then scuff it up with 80-grit sand paper. I install it using 30-minute epoxy, thinned with alchol and then I add milled fiberglass (Great Planes offers this) to give it strength. The thinned epoxy ensures great penetration in to the balsa around the wire, and the milled glass offers some additional re-enforcement. This technique works great and is light.
Hope that this helps!
Kevin