A few years ago at NSConference, I bought an Incase MacBook Air 11" sleeve off another attendee. I'd been unsuccessfully looking for a good backpack to hold my MacBook Air and not add much bulk. So I had to make one.
This is not a very complicated task that needs much description. All I did was buy a roll of backpack strap, a quick release buckle (so I don't have to wind out of it when its inconvenient), a matching tri-glide slider, and a shoulder pad.
I took a needle and thread and manually sewed the strap to the upper left and lower right corners, making sure the ends were at an angle so that they'd come off the backpack straight. I had to do several rows of sewing to make sure it was attached strongly enough to hold the weight of not only the MacBook, but also the power brick.
Now while I wish I'd had a sewing machine at the time to make the stitch nicer, you don't usually see that side of the backpack, so at least there's that consolation. Once I'd looped the strap through the pad and slider and sewed the end that went around the slider's middle bar onto itself (to get me adjustable strap length), it turned out to be strong enough for not just the MacBook and power supply, but also an iPad, plug adapters and a 3G dongle.
I wonder why no backpack manufacturer makes small holsters like this. I'll probably add a zipper to the outside pocket of the sleeve, but apart from that it's a very useful bag now.
NB - The photos are of a MacBook, but I used to keep a MacBook Air 11" in there and can assure you that it fits just fine, though the bulkier power supply requires a bit of thrusting to get it in the outside pocket.