Let me know when they actually coalesce. But, really, there are better options than a wall. Sanctions on employers that are actually enforced. (Of course, that would hurt The Donald). Deportation for any offense. Stop giving benefits of any kind to illegals.
Just have Congress pass a law annexing Mexico.
Within days, the Mexican military would be on our border as a show of force to prevent the annexation. They wouldn't allow free passage out of Mexico, as that'd be seen as official Mexican support of American annexation (ie: free movement between US territories).
Thus, the border is secured... by Mexican forces.