Piece of rubber like from an inner tube repair kit and a clamp. If that doesn't work someone will be along pretty quick with more suggestions.

Hmmm, the direct pressure solution. Yes, I suppose a hose clamp or two, plus rubber would work.

My solution was to wrap it with a bicycle innertube pulled nice and tight, followed by two hose clamps. Now to see how long it lasts.

And now I "seemingly" recall being suspicious when I installed the basement loop and sweated this exact joint. While it took a lot of solder, I still should have applied a bit more when I had the chance. I think the very thin gauge copper of the baseboard (female end) was a bit "out of round" or something.

That's ACE Hardware.