Originally Posted by kuan

I have a very similar one which I use for daily riding. I even think it's the same dimensions and has the same pockets. It holds a small U-Lock and cable, map, a few bars, sandwich, an orange, a rain _resistant_ jacket, wallet/phone.

Some observations:

I have brifters on my bike and the cables cross on top of the bag. I have to work through the cables to open it.

I have it resting on a front rack. The rear wall is slightly floppy when it is not filled. A decaleur would be ideal, but then it would add weight to your bag which you said don't want.