Were it me, and with a nice light like that, I'd consider putting a lock-nut on the bolt on the inside of the rack mount, if you have the space for it.
Or even a half nut if space is tight.
Thanks - these are rack mounts that go into the seat stay, so that’s not really an option. But this isn’t a set up for being left locked up outside for extended periods.
I don’t much like racks, so trying to avoid the option of fitting one. But the bike has rack mounts, so figured I could use them to do this.