I’ve heard a lot about this problem with the U-locks. Their mounts are verycheap plastic and they end up useless after some rides. So I came up with this solution for my girlfriend’s bike (which has a very small triangle and the mount doesn’t fit). It works brilliantly and its very easy to mount and dismount on your bike. It’s actually just two bunjee cords holding the Lock from touching the rack and few pieces of inner tube to keep the hooks in place. You don’t have to take the cords out everytime you dismount the lock, just hook them on the rack.
Take a look at the pics bellow.