I've been using this case from Brenthaven. It's only $25 and fits the 11" MBA, protects it very well, has a shoulder strap as an extra option and has an outside pocket for charger, etc.
http://www.brenthaven.com/ipad-sleeve-cases#collections/trek-sleeve-ipad
They now list it for an iPad but when I bought it it was under 11" PC cases. I guess they thought it would sell better listed as an iPad case rather than a netbook case. It uses their CORE system, so you could dsrop this from five feet without damaging your MBA. Good value for only $25.
http://www.brenthaven.com/ipad-sleeve-cases#collections/trek-sleeve-ipad
They now list it for an iPad but when I bought it it was under 11" PC cases. I guess they thought it would sell better listed as an iPad case rather than a netbook case. It uses their CORE system, so you could dsrop this from five feet without damaging your MBA. Good value for only $25.