I'm looking at this "fuel tank bag", but it might be a tight fit: https://www.amazon.com/dp/B0C7G44F3V?tag=sce0f-20
That said, the Tusk Olympus bag (I think) from the picture in the first entry looks smaller yet: https://www.amazon.com/dp/B0C9PTHQ9S?tag=sce0f-20 Not sure how he got the...