I attatch mine to the side of my rack using a single rack strap.

How I did it was my particular rack (Avenir heavy duty) has a weird rear support design, where the supports form a diamond shape...so the strap does not slide down much, even if I'm being stupid and getting the bike airborne. So I just ptu the hook on the rear support, then wrapped it twice and put the other hook on the rear support....then just slide the lock in and out of the straps as needed. Doesn't even rattle unless I'm doing something I really shouldn't.