Semi on topic semi off topic, I use a slightly different alternative. I have an old biking hydration pack that I took the bladder out of, it's slim enough that even loaded I can put it on under my rain gear comfortably. It also is a breathing mesh fabric with an extremely narrow footprint on your back, it's not cool by any means but it beats any other bag I've used in the summer time as well. Light loads only obviously, i.g. a small document tube and a smaller kindle in my case with text books loaded on it.