For my first bag, my shootin' buddy & I first made a pattern from heavy brown paper - like from a paper bag and then transfered that to chipboard. [The kind of cardboard that used to be on the back of the pencil tablets.]
We looked at alot of different sources, as you have, for the styles and 'look' we wanted and then tried to figure out the size from the description and/or picture.
We used tape and staples to fasten the pieces together and we learned alot about the order of assembly and manipulating the different thickness of materials.
If you can, put the stuff you will put in the real bag into the mock-up. See if your hand fits, maybe a slight change in the curve will fit the priming horn better - maybe the bag needs to be deeper so you can hang a knapping hammer or short starter inside the bag.
Make your second bag fancy.