Look for sued leather in the goodwill store near you. Purses, coats, skirts, etc. come in suede. Or you can buy a chamois cloth at your auto center, which will be light tan, and sew a piece around the sling. If you buy some of the synthetic foam padding available at hobby stores, you can actually so a wider " pad " covered in the leather( either the Chamois, or the suede) to fit the sling and make the sling wider, to carry more weight comfortably. Once you have the chamois cut to size, and before sewing, just use a good stain you can buy at shoe repair shops, or from suppliers like Tandy Leather company. By using sued, or Chamois, you give a soft cover to the synthetic pad, and that held hold the sling in position on your shoulder as you move about.
I use a sling backwards, and carry my gun muzzle down. That means my wider portion of my sling( the commercial sling was made with a wider strip with a suede backing sewed to it) is closer to the buttstock than to the forestock.
I carry the gun muzzle-down, to keep water,( rain, sleet, fog, mist, etc.) out of the barrel, and to keep the long barrel from hitting branches overhead. I control the gun with my hand on the forestock, lifting it up over brush and logs when needed. I can also lift the whole gun off the sling if crossing through tall brush or climbing through a hole in vines and branches. The muzzle goes through before I do. By carrying the gun upside down, I also control the lock, which is at my side or under my arm, so sticks, and branches can't trip a trigger or cock the hammer. I am not fond of carrying any rifle loaded, with a sling carry over my shoulder, muzzle up. You have no control over that trigger and hammer, and the muzzle is simply way to close to my ears, no matter how long the barrel may be. If the gun were to fire accidentally, the muzzle blast will probably destroy your hearing, permanently, in the near ear, and the muzzle flash will probably set your hair and hat on fire, too.
If you remove the percussion cap, or empty the pan of flash powder, and lower the cock, with the frizzen carried open, a gun CAN BE carried safely with a sling carry, muzzle up. When you transport a gun in that carry, it may as well be unloaded, as you are not going to bring the gun to bear on any target- human or game, quickly enough to do much good. I can say the same with the over-the-shoulder carry, and, for most people, who don't practice, the trail carry, and the elbow carry.
If you Google " slings" you might find pads for slings you can simply buy. Look at what Bob Allen has, as they cater to the high end shotgun crowd with lots of leather everything. Brownell's would be another possible source. :thumbsup: