I'm not sold a national emergency has to be declared.
Look at the military and homeland/national security apparatus and direct them to secure the border.
Every defense-related department chips in.
Army, Navy, National Guard, Marines, Coast Guard, Air Force, CIA, FBI, Homeland, DOD, NSA, DIA, etc...gets a hundred miles or so.
Move all those headquarters down there and build some military bases which will have to be secured and impregnable, run some war games with the Army corps of engineers in which they practice building walls...