This messenger bag closes with, among other things, a buckle and lid. Maybe not a good choice if you prefer lots of colour choices, but it does come in black and it has an 11-litre volume.
From Joom
BagBase Two-tone Digital Messenger Bag (Up To 15.6inch Laptop Compartment)
OnBuy
(One Size, Black) BagBase Two-tone Digital Messenger Bag (Up To 15.6inch Laptop Compartment)