Think Tank Photo Retrospective 7 shoulder bag. Shown with two Leica M cameras; L-R: Leica M9 with 50mm Noctilux ASPH and M9-P with 35mm Summilux ASPH. May 31, 2012. Photo: Edmond Terakopian
Think Tank Photo Retrospective 7 shoulder bag. Shown with two Leica M cameras; L-R: Leica M9 with 50mm Noctilux ASPH and M9-P with 35mm Summilux ASPH. May 31, 2012. Photo: Edmond Terakopian
Photo This & That 
I have mine enclosed in an Acme sleeve which does the job. I will say I take my iPad more often than the MacBook Air most times. One good thing is the raincover will protect the Air while in the front pocket from this lovely summer weather.
I’d suggest you take your 13″ Air with you and try it in the bag to settle your mind regarding whether it would work in the way you want it too.