On one bike, no longer in my possesion, I attached such a lock with hooks that had bolds on the other end. I used normal nuts to fix the lock to those hooks.  

As it was about 30 years ago, I do not remember how and where I got hold of those hooks. I do remember that I used tape, around the frame on that point, to protect the paint and to make for a better fit.  

If you feel the cable ties work for you, keep using them. You will likely not lose the lock unless all 6 fail at the same time. When the lock starts to rattle it is time to check and likely replace the ties. If you re-do all of them, you may want to tape your frame to protect it against the rub of your lock.

For longer locking, add a second lock with which you can fix your bike to a sturdy object. Your lock is made to add a cable and you enter it in the top of the side that is in your photo. Those come in several lengths and the longer ones seem useful. I use a separate cable lock and a friend a D lock instead.