The next time you cast lead, put some in a tablespoon and drop a rattle in it to see how long it takes to wreck it. You might be able to super glue the rattles right to the wire forms and pour your lead so the rattle is semi-enclosed. The lead in a mold will lose its radical heat fairly fast and the rattles may tolerate it. Never know, eh?
Adding the rattles after casting is probably better done on the back or front, on the side I think offers too much surface to allow them to be knocked off. I'd put them on with epoxy so you can puddle the glue around the rattle a little for more protection. If you are going to powder coat and cure these, add the rattles after the paint has been baked.