Luie, here's my bag which I use for all my stuff for camping, cooking and such, not shooting. I call it a haversack, but possibles bag would be OK, too, just a bit later. I went to the ladies fabric store and found some lightweight knobby canvas which could pass for homespun and made it from that. After I finished I painted it with melted beeswax to waterproof it, and that is still in excellent shape after 15 years. The bag is 16" tall and 13" wide, made from only one piece of cloth, folded as you can see in the pictures. Sewed it with flax thread. The buttons were made of a slice of deer antler. The strap is made from a long strip of the same cloth, folded along the length and sewn along one side, then turned inside out so the stitching is inside. The strap isn't treated with wax.
Notice that the flap is folded back so it is double. Also notice that there is an inside button which holds the front up when there is some weight in the bag. And, the bag was sewn along the sides and then turned inside out to put those stitches inside.
The bag is big enough to carry my corn boiler and cup plus my food when I'm trekking, big enough for my turkey calls, rain gear and such when hunting turkeys, etc. I generally don't wear it when hunting, but it works slick as a whistle to take my stuff to base camp or to carry special gear I may need on a hunt.
Mine is only one option, but however you decide to make yours, you will find it handy as can be.
NB Found this old picture of mine in action.
Spence