Some times you need to transport more than your custom design can carry. And who wants a plastic top box chasing you behind your back, when you spent 1/2 year cutting things of your bike? I found out that deckbags for sea kayaks got all the detales you need. They are water proof, slim and got a lot of fitting options. The company www.chillcheater.com got a deckbag of super quality for a good prize. Comes in many colors, and with a textile surface.
