Silver solder the swivel to the rod pipe. A surprisingly small contact patch will hold if it hard silver solder.
Helps if the rod pipe is silver soldered to the rib too. Use the second pipe not the upper one.
This is traditional size but rattles with a modern quick detach swivels.
I pin them as well as solder to help keep them in place while heating.
This was accidentally "antiqued" with some idiotic use of nitric that was just a little too strong:
But it fit the rib good so I will live with it.
Dan