Originally Posted by PaulRivers

Keep in mind that the 140L is designed to be mounted to something that's the shape of a handlebar. If you can wrap something around part of the rack so it's the width of a handlebar, the light would mount directly on the rack (well kind-of-directly). Which Tubus rack do you have? Most racks have a bar across the back, if you wrapped something around it and you don't need to pull your bag off your bike backwards (like the Topeak mounting system), you might be able to put it there.

I'm not saying it's not a pita, just mentioning it if you have to create some sort of custom mount anyways. What exact tubus rack do you have?