Not a rack pack I know, but if you're after small and handy storage solutions, try a pair of Toureg handlebar bags from Hein Gericke...they're only about 8 quid each.
I've got one on the back of my Adv- it attatches to the grab rails perfectly, and the other is on my bar cross piece....I can open and close it on the move so on longer journeys I keep a bag of sweets or dried fruit or whatever in there, plus phone, visor wipe and loose change etc.
Also not quite what you asked about but try a cash box drilled and fitted under a top box mount on the rack....still lets you use and top box but it provides secure warerproof storage not just a zipped bag
Total price less than a tenner plus an hour or so drilling/measuring/mounting
Toerag bag in place (cash box in place under top box)