You're not gonna find much if any, that will accept a large case like the UAG. Since the S8+ is new you might be waiting for a while until someone makes one big enough to fit the phone with the UAG case on it.
I don't use armbands, but i can definitely recommend the Flipbelt. I've had the original one for a few years and used it to hold my previous Note phones, keys and wallet all at once.