Paul: I know that you posted your question a LONG time ago, but I did have a similar problem with my boom. It had worked itself to the point where the end of the boom was just slightly flared out.
At the local plastics stores, I bought a small, thin sheet of Ultra-High Molecular Weight Polyethylene (
http://en.wikipedia.org/wiki/Ultra_high_molecular_weight_polyethylene)
You can cut UHMW with scissors and its virtually indestructable to wear. I made three small "shims" that I wedged in between the boom and gooseneck fitting when I resinserted the fitting. I needed a rubber mallet to persuade the fitting back in gently.
After reinstalling the fasteners, the fit was as snug as you could hope for.
I've since replaced the boom (to solve a, er, slight "bend" at the vang), but plan to put a small UHMW shim in the new one to make up for the wear on the old gooseneck.
