Stainless pop rivet. It has to be a snug fit in the hole or it wont work.
Just buy a new boom, every little creek, crack, or slip that you hear while using it will make you worry, taking your attention away from having worry free sailing. "What was that? I may have to go in and check my boom, I hope I make it in".
Not worth it, believe me I've tried stuff like that. Back in the late 80's I had a mast that would make these cracking sounds whenever I rotated the booms up to rigging level (tie on booms), so I shoved a piece of broken mast up inside the weakened mast to where the booms attached, with glue. It stopped the cracking noises while rigging, but I never trusted it, and got rid of it, not to mention the added weight.
It was not worry free sailing.
Also, what caused that kind of unusual wear? Something did, has that been addressed? Or are you just patching up something that shouldn't have happened in the first place? If you aren't sure what caused it, you're just wasting time.
Easy solution if you have a spare $1500 laying around which I don't, as it is it cost me nothing, I enjoyed spreading windsurf stuff and carbon fibre over the kitchen table to repair at it night and I feel a level of satisfaction and accomplishment having done the repair and getting decrepit's approval.
I tested it yesterday in a 25kt Northerly with my 9.5 RB sail all went well until the uni-joint broke. Glad that happened now and not during the Raceboard Nationals in a few days.
I mostly use large Raceboard gear, using it week in week out it needs maintenance to keep it going. Just replacing stuff everytime a problem develops is not practical or affordable. That said I agree mast's need to be replaced if they ever fail.
There are good used booms that are in much better shape.
I just don't like seeing people take chances like that.