My Tardis bike bag is pretty minimal, but still does a good job of protecting my bike when padded with my clothes and other gear. A thing nylon or poly bad would be lighter and pack smaller but I like the Cordura material of the Tardis as a good protection vs weight compromise. I can go on buses, taxis, trains and planes with it and can even carry it on my bike in a pinch and it's small packed size allows it to be cheaply posted and easily stored.
the Tradis is under the flap of my saddlebag
