Minto,
May be a blessing in disguise that the fin broke off when you tried to lift the barrel. Had you been riding and it broke off on the ride you would not have the piece to weld back on. Also, the cracked piece in place would be no help to the welder, as it would need to come off anyway in order to be chamferred to accept the weld bead. I'd skip trying adhesive because it could still come off and get lost. Though, if you wanted to try adhesive, I suspect JB Weld would be the likely candidate. You could drill a very small hole in the fin and add a fine safety wire that saves the fin if the adhesive did not hold. You could just run without the fin until the next time the barrels need to come off, which , we hope, would be after the world returns has returned to us.
Richard L.