I experimented for hours until I found an acceptable solution in my case. I have one of the newer OnGuard U-locks which have a complex mounting bracket. Well after concluding it is not ever going to fit inside the main triangle, I finally mounted the bracket on the seat tube (note: not the seat post), directly above the top tube. I removed the seat post clamp to slide the bracket on the short piece of tube, after which the clamp goes back on. It just managed to fit.

The bracket sticks out at a bit of an angle such that the U-lock is on the left side of the frame, parallel with the frame, hanging down, and a small length of velcro tape fixes the U section to the seat stay so it does not rattle and it reduces the risk of the whole thing breaking off when going over bumps. My left foot just clears it while pedalling.

Sorry, no photo unless you are desparate.