My first gun sleeve was made of dark brown pigskin leather I bought in 1972 from a store in San Francisco. It worked well; however I thought a waste of leather.
Since then I have made my gun sleeves from two layers of cloth. The inner layer is of flannel and the outer layer is of tougher material such as cotton pillow ticking or upholstery fabric. You can use colorful or drab material as you please to match their intended use, and to make it easy to identify your gun in the rack or leaning against the building or tree , etc. as necessary.
After cutting both fabrics to size, I match the "right sides" together sew the three sides, then turn them inside out , fold in half lengthwise ,and sew the "nose" and long side together leaving abought eight inches open on the long side. I attach some ties of woven tape to the open corners , and hem the opening.
These have worked well over the years. They are cheap and protect each muzzleloader from scratches.