At a church where I play handbells there is a bucket full of lagerphones which have the bottle caps in pairs, 2 pairs to roughly a 1 3/4" nail if I recall correctly, assuming 1/4 to 3/8" of the nail is inside the wood. I think the crimped sides are facing in each pair but I wouldn't swear to that--mental notes simply aren't what they used to be.

They put 4 nailsful of caps about 2 inches apart or less, just enough clearance between rows for good rattling. How many of these courses of rattles depended on the length of broom/mop stick, particularly the mop sticks have wacky faces added, and where the "hair" hangs down of course there can't be rattles, they just'd tangle in the mop and go silent. You want the hole in the cap to be made with a bigger nail that what it'll rattle on, so as to be loose enough to move freely.

These ones have the lowest foot or two cap-free, sometimes with decorative streamers. There is also a cap-free zone below the broom or mop-head (call it the neck?) where the stick is held.

Some were hockey sticks that lost their business ends, probably easier to nail into, they don't roll.

The ones I've seen look for all the world like New Year's Eve noise makers. Have fun with it, JJ!

~CH.