Here's another angle, showing my bulky waterproof Chrome backpack. It just barely fits in the Donkey Boxx when loaded with textbooks, food, rain gear and everything else I need to carry. And having a permanently affixed basket is great for impromptu shopping.
Whenever I can't fit the backpack in the Donkey Boxx, I put some stuff in the box and put the rest in the backpack and just wear the backpack. But 90% of the time my backpack fits in the box.