Like JustTubbs mentioned I also recommend the 20L Dry bags. This summer I did a 11,000 mile round trip from Charlotte, NC to Alaska and had three of the Sea to Summit Bags on top of my Touratech boxes. They are very tough and reliable. I do recommend the Sea to Summit Big River Dry bags because they have four sash points attached to the bags which make it easy to attach to your boxes. Touratech also has straps available that make attaching the dry bag very fast and easy.
Whether on land or water, keep your gear dry and functioning with the 20-liter Sea to Summit Big River dry bag. Fully taped seams and a hypalon roll-top closure offer reliable waterproof performance.