A solution in search of a problem. I carry my U-Lock like this.........



............hooked through the rails of my saddle and steadied by my seat bag. Someone pointed out that the lock might swing against the seatstays and chip the paint, so I added a pair of 1-1/2" strips of black electrical tape on the seatstays to protect them.

It's silent, secure, and the seat bag is squeezed just enough by the U-Lock that the lock is mostly prevented from swinging anyway.