Craftsman weedwacker trimmer, model #316.791020

This trimmer is designed to interchange for other purposes. Because of this features, my trimmer does not have a tight secure head string cutter. The head "bobbles", very annoying. The only thing I can see to do is drill a hole through the shaft to drive a bolt through to tighten it up. I don't know how this will affect the cable that runs down through the shaft?

Anybody know what I am talking about? Any other suggestions? I could send a pic for your review. Thanks