I had an acquaintance who hunted Africa a lot and he kept borrowing one of my canvas web slings with hooks that I had picked up in London to take on his safaris. He finally talked me out of one. He said leather slings were too hard to keep from getting moldy under field conditions.
That is good to know, would not have thought of that. In my case, I have no plans to hunt in Africa and the conditions here during hunting season is not very friendly to mould (almost always below freezing). Another option would be "mill felt". It is 100% mould proof extremely tough and even mice won't chew it. However, it would LOOK more at home on a $75 musket made in India than an English SXS LOL.