I've made some excellent bags using canvas, and lined them with denim patch material. That is very inexpensive, and you can and will make several before you settle on a design that works for you. I like what is sometimes called a hanging pocket on the inside to the back of the bag for the balls. That keeps them up off the bottom of the bag, where other tools and such lay, and makes finding and removing balls much easier. On flint rifle bags, I also like a pouch sewn to the outside of the back of the bag, for my flint-knapper/turnscrew.
On a Hawken bag, I sew pockets to the front of the pouch separated by a centre seam. One is for my capper, and the other is for my pre-cut and lubed patch container. On all of my bags, I sew a leather tube on the front of the strap for my starter.
Shoulder strap: make it short enough that when you put your hand into the pouch to retrieve something, you still have arm extension remaining. Otherwise, when you are digging for an item, you'll look like a dog chasing its tail. I hand my horn on a separate strap, and suspend it above the pouch so when I lift the lid of the bag, the horn is not in the way, and so that when I have to move quickly my arm pins the horn to my side under the weight of the rifle in my hand.
TC Albert wrote a nice book on pouch making ...
