If You Want A shell
Then Speck make good stuff, I have the clear shell's for my Air and my MBP....If you want something that doesn't attach to the actual device then a sleeve would seem to be your best bet.
Remember that having the shell on won't prevent you from pulling your Mac out of your backpack, and a sleeve only protects your device when it's in it.
If you don't want scratches etc. I'd go for a shell.