Howdy, Pard,
What follows may not be the definitive answer you are looking for, but I will give you my opinion and what I use:
According to Stephen Dorsey's excellent references, most of the 1870's era waist belts, sword/saber belts for enlisted men were made of "collar leather". Unfortunately, Dorsey does not describe what that term means. It might refer to leather used in horse collars, although there is another term for "bridle leather". It also depends on what belt you are making. I use 8 - 9 oz. (1/8" - 9/64") thick leather for the 1-7/8" wide waist belts and sword or saber belts for enlisted men, with either the Pattern 1851 Eagle belt plate with applied silver wreath or the M1874 "US" belt plate.
Note, that many officers who were CW veterans preferred the enlisted belts and Eagle plates with the silver wreaths, rather than the gold-washed plates with the matching wreaths and much lighter, folded construction belts. However, during the Indian Wars campaigns, I doubt many officers wore the U.S. plates. They probably used the 1851 Enlisted pattern belt plates. Hope this is of some help.