[Iron Matron]

But electricity (or some physical deterrent) would remove the weak link of requiring a human response to all attempted crossings. <<<

Electric fencing has worked for cattle for years. For Mexico, I would recommend electric fencing topped with RAZOR WIRE, surrounded by a mine field.

[RS]

"Electric fencing has worked for cattle for years."

Uh... Cows haven't yet figured out how to throw an old mattress over the wires.