For the longest time, I was searching for the perfect rack trunk. But they were always too small for what I needed to carry, like changes of clothes, lunch, laptop and repair kit for comuting to work.
I tried duffel backs but I just didn't like the bungee cord method because you'd have to undo it just take out a jacket or something. I wanted to look for bags that already had side loops so they could strapped or bungeed down without interfering with any zippers. Nobody makes bags like that. Even mounting a knap sack was cumbersome because the shoulder straps would dangle all over the place.
Finally, I settled for the Arkel combination pannier/backpack. It's a pannier that's big enough for all the things I needed to carry and when you take it off the bike, you unzip to take the backpack shoulder straps out. It even has a loose pocket to carry your helmet.