I just got one of these neat little packs that are custom made to fit on Renntec racks. They are well made and just the right size for commuting or popping to the shops. I can get a loosely folded shirt, my waterproofs and other bits and bobs in there, plus if needed it is expandable and is big enough to take a full face lid. It attaches securely to the rack with velcro and a buckle and detaches in seconds, 'comes with a shouder strap and handle, 'also has a water-resistant lining. I'd been looking for something this sensible and convenient for years.
Best of all, these used to be £55 but are now twenty quid delivered. Reccomended.
http://www.ebay.co.uk/itm/ws/eBayISAPI.dll?ViewItem&item=120908783180&ssPageName=ADME:X:AAQ:GB:1123
