I'll bet the waterproof cover that Arkel sells for their panniers would work on your backpack as well.

Yellow, high visibility, works great. Takes about 2 seconds to cover something or take it off. Costs about $15. Check the Arkel website.

(I use the Arkel waterproof cover on my Arkel panniers -- but there is nothing that says you can't use it for other similarly shaped items).