I think you might find that the terminology of "possibles bag" is a might bit obscured. While it is ofter the term used for the bag to hold the shooting supplies, you'll find that the bag used to hold shooting supplies was referred to as a "hunting pouch", "shooting pouch" and possibly several other terms.
A "possibles bag" was just for that . . . "possibles" that you might need. Not o start a big "hoopla" - just stating that the two terms are often interchanged. To me, a possibles bag is not different that a haversack - larger than a hunting pouch to hold such things as (originally) jerky, parched corn and other food items as well as possibly such things as a fire kit, sewing kit for patching clothes and moccasins, tobacco, etc. The hunting pouch carried the bare necessities for loading the rifle - balls, patching material, a spare flint or two or percussion caps possibly a small powder flask if a horn is not carried, etc. - possible a knife attached to it with a sheath for cutting patching material at the muzzle and general use, possibly a small trail axe attached to the back. Anything else was"excess baggafe". I have had the opportunity to see and examine quite a few original hunting pouches and for the most part, those of the 1800's were small and if a horn was attached, it was usually a "day horn" which held enough powder for a day's worth of hunting. Earlier, when hunters, militia, etc. were required to be in the field, the horns were of course much larger as were the bags in many cases.
Texas Reb - there is no "right or wrong". Determine what your requirements are and that will determine the size of bag you need. I can guarantee you that if you start with a large bag, chances are that you will eventually move to a much smaller "hunting pouch" and weed the unnecessary contents out. Personally, I would think you would be better served to carry a smaller "hunting pouch" to cary the minimum things you need to carry to load and shoot your rifle in the field, and then have a separate haversack o "possibles bag" to carry all the other stuff - water bottle, first aid kit, food, etc.
One of the tarred Civil War haversacks would work well for that or similar. Canvas bags are fairly easy to make if you can see. Pillow ticking, canvas, etc. can easily be used to construct them. If you are hunting or on the move, make your haversack so it can have an adjustable strap to conform with how many clothes you are wearing. It is much easier to carry a haversack with it riding up higher more under your arm than it is to have it banging against your hip every step you take. The same with a hunting pouch.
If you decide to make your own pillow ticking or canvas pouch, when it's all done, it can be painted with a latex paint and the paint will soak in and help to make it water proof. If you want to "age" it, once it is thoroughly dry and you have your better half's permission, throw it in the dryer for a half hour or so. I used to make them and use black or tile red latex paint on them for a period look.
Again though, there is not right or wrong - it's up to what works for you. I can guarantee you that if our ancestors looked into most of the hunting pouches that carried today, they would go through them and toss half of the contents into the ditch along side the road as being "unnecessary clutter".
Good luck to you.
Hawg - that left handed lock in your photo and the stock as well looks like you have an interesting project going!