Carrying your computer in your hand can be cumbersome, use a computer bag instead. There are several ways to carry this bag, one of them being a detachable shoulder strap.
From Quzo
DICOTA Slim Case Base 11-12.5 31.8 cm (12.5") Messenger case Grey
Technoworld PLC
Dicota Slim Case Base 11-12.5 notebook case 31.8 cm (12.5") Messenger case Grey
OnBuy
Dicota Slim Case Base 11-12.5 12.5 Messenger case Grey