What style, size, color, flap design etc does everyone look for when deciding on a bag to keep all the necessary ingredients for the care and feeding of a muzzleloading firearm?
I made my first "Hunting Bag" in 1972 solely from pictures in books, because I had no mentors and information was darn hard to find for me while stationed in Oceanside, CA. Buckskin was not available at the local Tandy Store, so I settled on buckskin colored cowhide split leather. Though I wound up with a functional design, the leather was so soft it folded in on itself and proved useless for the intended purpose.
So the first thing I advise is choose your leather well.
Fortunately before I made my second "Shot Pouch" a few years later, a friend and mentor took me to Friendship, IN for the Spring National Shoot on the Primitive Range. There I saw a myriad of different period designs, but also got involved in The Northwest Trade Gun matches where we shot both round ball at targets and shot at flying clay pigeons, threw hawks and knives and had to start a fire by flint and steel in less than 10 seconds. The rule was "once you cross the line, you can't go back for anything you need," just as it was on the frontier. Over the years of shooting that match; I saw guys who forgot balls, wads, tools, etc., etc. and they suffered loss of points in the match because they didn't have what they needed when they needed it.
So the second thing I advise is choose carefully what you need to carry and make your bag big enough for those things and your hand to get into the pouch, comfortably.
OK, as to features:
The strap - I've made Military and a couple civilian Pouches with one piece straps. I DON'T like one piece straps where I can't adjust the length of the strap for different amounts of clothing worn between the seasons. I prefer to "let the strap length out" when I'm wearing thicker clothes in different seasons. So that means I will use a period correct buckle and make the strap from two pieces.
It seems during the period, they had the buckle on the side of the strap that faced forward more often than not, no matter which side of the body they carried the bag. I can only guess that was to make it easier to adjust while it was/is on the body. Personally I've found the buckle on the forward strap to be a bit of a PITA and especially if you wish to mount a patch knife on the forward strap. I've also found the buckle can hang up in brush when worn that way. So I always put the buckle on the the strap that faces to the rear when worn on the body. I don't mind taking the pouch off a couple times when I let the strap out, until I get the correct length for thicker clothing, as it will stay that way until the weather warms up.
The Pouch Flap - My favorite Shot Pouch has a folded over flap of one piece with the back of the pouch. However, the pouch is made of thick leather suitable to be used by a Militiaman with the removeable wood cartridge block. Since most folks won't use that thick of leather, I believe it is better for many if not most civilian pouches to have the flap as separate piece and sewn to the rear piece of the pouch. Edge banding where the two parts are sewn together are a nice and useful touch. I may or may not put edge banding on the flap, depends on what I'm trying to recreate. Oh, I also prefer a period metal button to use to secure the pouch flap when I'm not loading or shooting. That keeps things IN the pouch when moving up and down hills or through rough country.
Inside the Pouch - Since having used a pouch with and without a divider, I would never again make or accept a pouch without one for myself. I prefer to put my balls and patches normally loose in the front part where I don't need to look in the pouch when target shooting or hunting. The now empty ball pouch goes in the rear while doing this. If I'm shooting shot, then I put a can containing wads and patches in the front along with a small pouch for shot. IOW, I put what I need to shoot in the front of the pouch and put tools and accessories in the rear of the pouch. There is not much period documentation on sewing leather pockets to the inside back of the pouch, but I also like to do that to better organize things in the rear of the pouch. Though another way to do that is make a leather "tool" wallet and store that in the rear of the pouch.
As far as D or O rings or Square rings on the straps, I don't use them unless I'm copying a specific and unusual original pouch. I put the strap on my body or the body of the customer and mark where the ends of the straps go (at outward angles) so the strap lays nicely against the body.
I prefer to use welts in many of seams to make them stronger and give a better look.
I hand stitch my Shot Pouches/Bags both because it is traditional and such stitching lasts longer than machine stitching.
Can't think of much else, though I may have missed something.
Gus