Okay y'all, Ive seen this come from my own work bench. This belt (that is shown) was laying flat when it was glued together then sewn. Or it was sewn on a flat table. Either way it was flat when it was constructed. the quality of leather doesn't matter when building a belt that only has one outside and one inside. When starting the dual leather belt either place it on its side in the shape of the letter O, start in the center and work towards one end, then return to the middle and finish adhering the other half. I have the Cowboy 4500 and i lay the inside over the arm and start in the middle and work to the end and then return to the middle and attach the other half, I also use wax paper to keep the contact cement separated until I'm ready to place the sides together. While I'm sewing the glued sides together the "belt" stays curved or arched.
Much like the way the inside of a wallet is smaller, the inside of a duel sided leather belt is slightly smaller. It doesn't matter how good the leather is, ti just needs to be placed correctly with the right amount of thread tension. Try doing it using the arch method, Glue it together using a hose hanger or the side of a waste paper basket. If you don't do this { or some form of it) you will have a dimpled belt interior!