Custom Designed and 3D printed U-lock Holder
I designed (in FreeCAD) and 3D printed (FDM, natural PLA), a custom U-lock holder that bolts onto one of the water bottle holder mount points on my bike, all without interfering with the water bottle in the water bottle cage on the other mount in my frame.
There's a 6° angle on the U-lock holder so that the lock doesn't physically interfere with the bike frame when inserting and removing it from the holder.
Didn't like my options before (carrying the lock in a backpack or locking it to my frame where it slowly damages the frame's paint and rattles around), so I custom made this to solve my problem.
Design idea is based on other 3D print model designs I found online. The difference with my custom one (other than that it fits my specific U-lock) is that it allows me to also keep a water bottle inside my frame with my second mount, whereas the models I found online physically interfered with the water bottle position.