If you don't think the tuners are snug enough you can wrap the shaft (B in the pic) with aluminum or copper tape (what you'd use for shielding). It's stiff and won't flex, so should work pretty well.
That said, I have some that are not that snug and don't rattle at all. When in tune, the string pulls the tuner snug against the wall of the hole, so it should not cause a rattle. The part of the tuner that does often rattle is the nut (A in the pic). I spent a couple of weeks looking for a rattle in my acoustic, only to discover that I just needed to tighten down the nut on the tuners.