Bike U-lock holder to bike top tube.
I use this with Kryptonite New York Fahgettaboudit lock.
The bike I use this with has a 31.8mm top tube.

Original FreeCAD files included, so you can adjust dimensions if needed.

Version two works much better as the loops are interlocked. This one can be tightened to stay in place much easier even with heavy locks like the one I have.