Building "The Wall" is perhaps a knee jerk reaction to the problem of migrants all over the world and one needs to look back down the line to try and address the reason why they are moving.
However, I have just come across Flight Rail Corp in California, a very small company without billionaire back up who have designed an atmospheric rail system that could change how people and freight are moved which is both simple to build and to run.
My suggestion would be for the American government to fund such a rail system along the border whereby it can move people and equipment easily and quickly and as the rail system is on stilts could easily be rigged to detect anybody passing underneath.
This would have two effects, it would provide security and more importantly demonstrate to the world a system of travel that might have the same disruptive effect as the mobile phone.
So Google the company to see just how innovative some people can be in America.