There are some backpacks that are designed with aerodynamics in mind, such as the the Ortlieb Flight. I have this bag, and it is thin enough that it doesn't get any wider than my waist as well as being shaped aerodynamically even if you are down in the drops. It may not be large enough for you though. If you are interested, I wrote a review on the forums a while ago. I agree that a tightly fitting messenger bag may work well too.