Loose bits

("Loose bits" sounds like a personal problem that I should see a doctor about, I know!)

I have a King 1250 sousaphone, and the bit is very loose in the neck. I have to hold it in place with my left hand pretty firmly, all the time. Anyone have any suggestions for what I can do? It would be nice to have my left hand free to hold onto other things, like music, or a beer.

Thanks!