Doesn't get much simpler than the Lyman bag,and add straps as seen here,
https://www.muzzleloadingforum.com/threads/hc-bag.118354/
Essentially a large rectangle folded in slightly unsymmetrical thirds (smallest "third" form the flap), with a smaller rectangle inside, and sewn along the edges.
Another good simple one is a bag documented to Colonial Virginia that is also a simple rectangle with a divider only sewn on the sides, but it is "turned" which provides a slight bit more volume. It uses a long one piece strap sewn to the back of the bag at one end and attaches with a button and a series of button hole in the strap to adjust for length at the other end. Hopefully someone can provide a link to one or more of the various articles about the bag. Here are some picks of my version that I can send you a pattern for.
View attachment 21596 View attachment 21597 View attachment 21598
I made it a little more complex than the original, with a thin piece of leather sewn over the top edges where the flap is sewn to the back, and I had to sew fabric to the back of the strap as the leather I used is too stretchy to work on it's own as a strap.