I remember that my dad used to cut leather laces for boots and snowshoes out of a piece of leather. I think he just drove a big nail into a spot on his old workbench and positioned a sharp razor blade next to it at the thickness he wanted, made a small notch and pulled on the leather. He made it look easy and my recollection of how he did it may be a bit fuzzy. But if you want an authentic looking leather one, maybe something like that would work. I did buy a leather belt several years ago and I remember the tech telling me that they needed to be adjusted to the tension you needed to treadle and also that I would probably have to adjust the tension in a bit as leather does stretch a bit. I think the modern ones not made of leather are easier to use but I currently do not have working treadle.