Thinktank Shape Shifter
The Thinktank Shape Shifter is an expandable backpack that can expand to hold a pretty substantial system, but compresses to less than 12cm. You can use it squashed down, or open it up to use it as a mid-sized backpack for larger equipment. Stores well, too, if you're not planning to use a backpack every day. Solidly made, in excellent condition. Interior dimensions when expanded are approximately 35x45x15cm. There's also a dedicated laptop compartment which should hold pretty much anything up to 15" or so (and possibly larger).