Previously I'd experimented with another tail-bag concept, but it proved too small and fussy. The Scully duffel is a great size: large enough for tools, rain suit, lunch, shoes, and other odds and ends. It's just the right width to be fully supported by my custom rear rack (plans here), and the length doesn't interfere with riding position, seated or standing. Construction quality is top-notch and the zippers are chunky and easy to operate with gloves on. Concealed shoulder straps convert it into a day-pack in seconds.
For now the bag attaches to the rack using a simple bungie loop. This is likely to chafe through the material on a bumpy trail ride, so I will probably use a bungie net to secure it.
Update September 21, 2017
Despite being immersed in a mud-hole and scourged by days of torrential rain on an adventure ride this summer, the bag has performed flawlessly in keeping my gear dry and resisting abuse from rocks, branches, and tip-overs. It's been easy to access and the size turns out to be almost perfect for all my daily needs of rain suit, clothes, food, water, sandals, etc. I've since swapped from using bungies to locking straps for more security and speed in fixing the bag and other stuff to my bike.