Some carry " range bags ", and then they carry something smaller for hunting bags. So, it depends on what you are wanting to use the bag for. Its another tool, you see.
A ball block to carry 3 to 5 balls already patched and hung from your neck, or just stuck in the bag is all you are likely to need for any hunt. Throw a few extra balls, and dry patches or patch strips in your bag to gather dust in the bottom, and you should be set. Then you will need stuff for shot loads. Wads, and shot carried in some kind of container. You can either carry a powder horn and separate measure for shot and powder, or measure the shot and powder out and store them in small tubes, or cans in your bag.
I am used to a powder horn, and adjustable measure for powder, and a dipper style measure for shot. I don't happen to use even volumes of both powder and shot in my 20, but that does work for some, and saves you from having to carry that extra. I do carry a brass canister for my cleaning patches, and a small bottle of cleaning " moose milk". For the Round ball loads, I use prelubed patching.
Then, even though I might not need them in the field, I carry a flint purse with extra flints, and a lead flint wrap. I also carry a flintlock tool for turning the cockscrew, and clearing the vent. Then I also have a patch puller, and a ball pulling screw, and a separate screw driver that fits the lock screws on my gun. When I am doing a hunter's walk, or actually going out on a deer hunt, I clean all the extra's out of my bag so I am not draggin around extra weight. But, I still carry my range box in the car, extra powder, tools, and a range rod, in case my ramrod is broken. Going back to the car to get my gun working again is one thing. Driving all the way back home is quite another.
Put in your possible bag what you need for the shooting you are going to do. I have been know to not carry either my powder horn or my possible bag on a wild Boar hunt, preferring to carry pre-measured powder charges in one pocket, and extra balls and patches in another. I also have a " Belt pouch ", I think you can still find them at October Country. It is small enough to carry just what I need on a day hunt, and not weigh me down. It would be very easy to measure shot loads into film cannisters, or tubes, and carry them in that pouch, along with the wads I would need. A powder horn and measure, or pre-measured powder charges carried in a large pocket and some cleaning patches would do for a short day in the woods.
