i dont need all the space in a messenger bag and find a lot of them too bulky.
so i picked up a laptop messenger bag the other day for 20 bucks. more of a man purse really
but still looks "business".
anyway its for 11" laptops or less and its perfect.
i can put 2 tubes, levers, food, drink, my shoes (sz11.5) and change of clothes and its fine.
it also is much neater and stays better with less work, on my rear rack.