Look at their Bikeblock or Megablock. I use the Bikeblock for my U-locks without any problem. I have attached them to the top tube and the seat tube. I don't know how well they will work on a rack though. You might be able to connect a portion of seatpost or PVC to the rack, then the Bikeblock to that.

I have small bikes, so often the U-lock won't fit inside the triangle if I have a water bottle. What I do is place the Bikeblock on the seat tube with the U-lock facing backwards, resting on the seat stay. I then velcro the lock to the stay. It's kind of difficult to place it correctly, but once done, it stays in place and no foot interference.