From Bike It this Waterproof Reflective HiVis Rucksack Cover fits most medium or small sized backpacks.
From Biketek this 15L Back Pack and Motorbike Helmet Carrier is a sensibly proportioned rucksack designed for motorcycle use. It features a unique built-in mesh helmet cradle enabling helmet to be carried without reducing main compartment capacity.
The 30L Biketek Urbano Motorbike Tail Bag Rucksack is of a contoured design with an expanding main compartment.