We already have agents on staff
Just post them every quarter of a mile with constant changing patrols
Then we need secured outposts and roads linking them with support. Trust me, I've done this for military bases overseas. Our southern border is similar. It all has a cost. The best approach utilizes a mixture on many different methods. For example, in some areas we should add ground sensors to identify tunneling. In other areas aerial drone support. In all areas, the manpower and equipment necessary to secure the border.