I'd respectfully suggest that your challenge with this project is that you're having to deal with a period of dramatic transition. The notion of the horn and bag set, as we understand it, was apparently just beginning to catch on, wrapping up several centuries of all sorts of other outlandish (to our eyes) methods of carrying ammunition. Wooden flasks, ornate metal ones, tortoise shells, stiff boiled leather containers, and hollowed out stag antlers, were all accepted approaches before the classic powder horn was widely adopted. I've even seen a documented early 18th century powder flask made out of a whole lobster claw! How weird is that? The actual horns that can be confidently dated to such an early era were, for the most part, flattened flasks made of horn with all sorts of metal hardware, often almost unrecognizable as a horn per se. As usual, of course, items most commonly used by poor people haven't survived to any great degree, so it seems reasonable to assume that it would have been common to 'cut to the chase' and use a horn in a relatively straightforward and unaltered form among those folks, since it's such an obvious expedient.
Bags present a similar challenge. Belt pouches of various kinds were common, as they had been since the early medieval period and before, long before they had anything to do with guns. Once carrying ammunition became the focus, all sorts of ingenious contraptions were developed, including separate ball compartments built into powder flasks and integral leather bullet pouches designed and built as part of the flasks. Plenty of period European outfits exist, and are depicted in art, that follow the general scheme of a belt-mounted bullet pouch with a permanently attached flask, often dangling underneath in a pretty awkward looking fashion.
I'd love to hear other thoughts on this, and wish you luck on your project!
