Ding, ding, ding. We have a winner. Thanks. I didn't know it was custom, but of course it is. It always is the most expensive things I like. Never fails. It is sweet and so is the bike.
Based on how perfectly it fits and the relatively simple construction i'm gonna guess it's a custom frame bag.
You could try to ask around locally if anyone knows someone in the community who makes custom bags - there's tons of small, independent businesses who do that kind of thing.
That's Indiana Schultz's bike, I recognize it. It's a Shaka Packs custom cotton duck frame bag.