I see what you mean, but it’s still my opinion that being confined between the barriers would not be the best option.
As for what to put between the barriers, how about some buried fougasse-type devices to spray intruders with something like skunk scent ,possibly mixed with dye? They could be deactivated if the patrollers had to enter the space between the barriers (though you might want to send the new guy in first, in case they were still active).
Right—and as I said and the link I posted has, they could drive inside the second one.
If it were me and I’d come all that way, a little skunk spray wouldn’t deter me. I guess I’m a little more ruthless in this situation than you are.
This, of course, would be huge: