We can never make anything un-stealable if someone really wants it they will get it. That being said, growing up in Miami and living in heavily populated cities I've found you take as much reasonable accommodation as possible and make sure they have to work to get it. My rotopax mount on Leitner's universal mounting plates, the most secure I could make it was mount more rotopax on the reverse side, this way when rotopax are mounted on the front and the back, and using the locks, there is no way to get to the bolts mounted on the plates because the rotopax cover them up and you would have to remove the locked rotopax to unscrew the mounting brackets.
As you can see, this is without the "inside" rotopax mounted, someone could use a wrench or socket wrench to undue the bolts holding the rotopax brackets, but if they did, they could steal the rotopax but they wouldn't be able to get them off the bracket because you would have to turn the bracket to release the rotopax. I see this as too much trouble, I also always keep the tailgate locked just to make it a little more difficult getting in there.
As you can see with the "inside" rotopax mounted, the have to mount horizontally, the diesel rotopax are the older style where you can interlock them, so I only had to use one lock, because once interlocked, you have to take them off together to get them off the mounts.
If you don't have room to mount more rotopax, you could always fabricate or re-purpose a hasp type lock that you mount using the rotopax mount bolts, then the hinge part goes over the bolts and use a pad lock to make it secure.
I've also seen folks just run a covered steel cable bike lock through the handles and around their racks to secure them more.
But honestly, I've had my truck and prior truck sitting in and around Austin for almost three years and no one has ever tried to steal my rotopax.
