I've had THREE nylon bellcranks bend or break. I don't trust them on anything above low speed sport planes.
You can't just use a simple number for tip weight. You need to size it the finished model including leadouts, engine, tank, etc.
For a 35-40 size plane I would test-balance it an add enough weight to achieve unity then add another 1/2 ounce.
Forget the box and drill a 1/8" hole and glue in some solder.