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.