I carry my MBA11" in Maxpedtion Sitka (S-style which is for lefties) one-shoulder bag with Tom Bihn Cache sleeve for extra protection.
There is a hidden 100oz hydration reservoir compartment that is perfect size for Cache/MBA. The MBA pretty much disappears in the bag I forget about it till I need it. Cache can stay in the compartment and you can pull out the MBA easily.
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Sweet Bag. I've been looking at those for a while. Would a 13" MBP - 15" MBP fit?