U-lock mounts tend to suck anyway. I usually just carry mine in my trunk bag.
You can also bungie it to your rack. On some racks, you can slide it in vertically, between the outer rail of the rack and the flat bed portion -- the U part will slip through but the thicker straight part of the lock won't, and it hangs there. Then you can put a thick rubber band on the lock, and attach it to the hook on the bottom of your rack (where a pannier would bungie on). The rubber band keeps it from bouncing.

Does that make any sense? If not, maybe I can post a picture.