Ask Mexico if they'd like to sell off Baja and Sonora. Arizona might get a coast out of the deal. Those living in the Mexican border states with a history of migration back and forth across the border would have free access into the US states. Mexico would get an infusion of cash. And the US would get control of the zone.
Okay, I realize that it might not be an easy sell. And perhaps it's not exactly what needs to happen. But we do need some out of the box thinking to deal with these issues in a humane and fair way.