Style truly not being a concern, only drop protection; I would recommend something like a military tank. Alternatively one of those big boxes used when moving filled with bubble wrap around the Mac, with a layer of foam cushions around that.
Protection while using or while carrying? No case protects very well when you’re using the Air. I have used backpacks for my laptops for years and they have always provided excellent protection. Padding on all sides that protect against drops.
Chances are you won't find one. In order to have that level of protection, the case would have to be significantly larger than the laptop inside it, which defeats the purpose of having a thin and light laptop to begin with. Such a case would by necessity add both bulk and weight, and that may mean the difference between your bag of choice holding the laptop or having to go up a size or two to accommodate the laptop and its case.