Concrete with stainless steel rebar throughout. Watchtowers every hundred yards manned by the military.
Maybe we should just invade Mexico and make it a US Territory. Our new southern border would be a heck of a lot easier to protect.
Honestly, I’ve wondered why the US doesn’t offer to help Mexico protect their border if they’re unable (or unwilling) to. If they’re unwilling, screw ‘em. Let’s do it anyway.