We would have to eminent domain all the private property along the border to create a buffer zone/national park/artificial wetland. It's a lot harder to tunnel under a swamp, and the crocs or gators would have a much better shot at long term survival. Honestly we should never have allowed development right up to the border in the first place, now if we really want to secure it, we're gonna either need the full cooperation of the Mexican government, or we're gonna need to annex a shitload of land on one side of the border or the other.