Mounting a U-Lock between rear-rack and wheel

A couple months ago, I saw someone who had made their own U-lock mount so they could carry their U-lock BETWEEN the wheel and the rear rack.

The mount was a narrow, metal tray the length of the U-lock with a groove in the top that the lock sat in. It was mounted with bolts on the rear-rack vertical supports. You mounted the lock by sliding it in from the back of the bike.The wonderful thing about this mounting position is that you can still carry panniers with the lock getting in the way.

I really want a lock mount like this for my bike. Does anyone have instructions/ideas on how to make such a thing? Or know who could make one for me?

Thanks!