mounts to a Thule roof rack. i dont know if it will work on anything else. i have 2, but the clamp doesnt work with my bike, so i replaced one of them. i am asking $60 shipped for the one.
this is with the rack in the position it would clamp to the down tube.

how it sits when not in use

pic of the clamp
the piece sticking up off the tray is broken off on the other side. i still have it and will include it, but it is broken. i am using a velcro strap to hold it down and it does fine.

it fits on my steel frame specialized, but not on my aluminum frame trek.