Would a double bag be appropriate for this era (1840s 1850s) and for use with a plains rifle of this type? The rifle is being made by W.B. Selb and will be a copy of a standard taper-barrel J&S Hawken taking 28 balls to the pound.
There is a two-compartment pouch shown on page 21 of
The Mountain Man's Sketchbook, Volume II. It really is essentially a "double bag, rather than a single pouch with a divider. Notes in the book indicate the original is in the Museum of the Fur Trade, and the pouch is made of black leather. The drawing in the book is about 3/4 size, so the actual pouch is probably about 8'' square, maybe a bit larger than average for pouches of that era.
Here is a period drawing by Rudolph Friedrich Kurz of an original pouch, horn, and Hawken rifle from about 1851. All of this kit belonged to Edwin Thompson Denig, who was the bourgeois of Fort Union:
That looks like quite a large horn, probably from a domestic bovine rather than a bison. The flap of the pouch appears to have a "tail," partially hidden by the horn, that tucks under a "keeper" to secure it. The strap arrangement is especially interesting... there is a wide, non-adjustable bandolier, with four shorter straps with buckles. The buckles on the smaller straps provide some adjustment for length, but also allow adjusting the relative positions of the horn and bag to each other, or even completely removing the bag and/or horn independently. Ingenious!
This old southern pouch and horn in the MESDA has the strap set up in essentially the same way:
For a more rustic look, this rig from a previous Cowan's Auction has always appealed to me, although I can't positively say it dates to the right time or place for you:
Finally, this pouch, collected from the Assiniboine people and currently in the AMNH, was more likely used with a trade gun than a Hawken, but I don't think it would be an inappropriate choice for you:
You might also look online for paintings and drawings by Karl (or Carl) Bodmer, Alfred Jacob Miller, George Catlin, or Charles Deas. All of them were accomplished artists who visited the west and saw plainsmen in their element.
We'll want to see that rifle, pouch, and horn after you've gotten it all together, you know...
Notchy Bob