I think it is a dumb idea and a waste of resources. I would prefer that we call back most of our troops over seas and place them on bases across the borders north and south and have them patrol.
A wall will not achieve its stated goal since they can go over, under, or through it. Now patrolled by 10k soldiers, with drones and sonar that would achieve the stated goal. Or at least it would invigorate local economies, lower our footprint in other countries, and provide both warm weather and cold weather training opportunities for our troops.