I've got one of these:
but it just isn't quite big enough. I can't fit a change of clothes in there along with a rain suit. I Would like something the size of a daypack backpack, and I'm thinking of just buying a pack with an internal frame and finding a way to strap it to the seat with bungee cords or something if I can't find bag that will otherwise work.
I tried my hiking backpack, but with a full load, I can't secure it high enough on the seat to keep it from resting on the fender, which then rubs on the tire.