I'm not sure if you're married to the idea of using your backpack, but if not, you might want to check out the $25 rack trunk that Schwinn offers at Walmart and other department stores. It's large enough for the items you described and it's got two side flaps that pop out when you want to add the convenience of panniers. I went through lots of different, very expensive carrying solutions before I settled on this. Saved me hundreds of dollars and works great.