A wall is fine, but it could be accomplished more cheaply.
US military towers every 500 yards, linked with all of the greatest communications and spotting technology. 2 persons occupy each tower - a spotter and a sniper. Orders are: shoot to kill. Signage everywhere, in Spanish and english.
no need for spotter and sniper, just set up automatic Century mini guns every 1000 yards and the check them weekly to reload.
Cool idea, but I’m thinking “Drones”!
You don’t even have to shoot them. Use the new pain ray guns. I hear they are very effective.