go for it!
the stitching doesn't hafta be as intense and close as what i did above. a simple overhand stitch goes much faster, is easier and does the same job. it's just a simple bag, and most of the remaining examples of 18th century shooting bags were just that - simple and small.
the weight of the leather only matters for how easy it'll be to stitch up. 4oz is plenty and will be stiff enuf for the straps. a half side (10-13 sq. ft.) is perfect, but smaller will work fine too because straps can be stitched together from shorter pieces. anything you make will be as "period correct" as it comes because there weren't no tandy stores back in the 17 or 18 hundreds.
make up a pattern from some stiff paper or light cardboard (free post office boxes
) - a front, a back with front flap, a tapered side piece, and a some 1" to 1-1/2" wide pieces to make the strap. for sewing up, i use that synthetic sinew (waxed polyester), but any stout thread will do just fine, along with a large needle. if the leather is thin, the needle might poke through it with the aid of pliers, if not the leather edges will need to get pricked or punched for the thread holes and a simple nail and hammer does the job.
now that yer admiring that new bag ya built, time to make a simple powder horn.