Pennies! Just don't tell the government. hehe Seriously that's what I've used. First, have somebody help hold the plane. Then stack the pennies out on the tip of the wing next to the last rib. Add enough so that the plane is balanced. This will tell you how much weight you have to add. Remove the wing and cut a small hole in the bottom covering out where you stacked the pennies. Stand the wing against the wall so the hole is at the bottom. Now just epoxy the pennies, in a single layer, to the side of the rib. This distributes the weight so that the rib won't break. Recover the hole and you're ready to go.

Just my 2 cents worth. yuck yuck

Kerry