Automated machine gun nests with overlapping firezones, infrared and tectonic sensors for guidance. Just have the border patrol occasionally drop off additional ammo.
Isn't technology great ?
I think we should let nature work for us. Erect concrete walls 50 yards apart, 20' high & slightly tilted toward Mexico, with a smooth surface that can't be grasped for climbing, but install concertina wire on top anyway. Make the concrete panels 35' wide so that 15' is underground to deter tunneling.
Populate the area between the walls with as many prickly pear cacti interspersed with bougainvillea plants that will grow and thrive; for fauna, plenty of rattlesnakes and Mexican cougars, with a substantial grubstake of all kinds of rodents and other prey animals for feeding.
Anyone able to safely make their way through all of that would deserve to be here.