It isn't as if we have never built a long barb-wire topped fence before. What is the combined distance of all the fences around US Airports? The one around DFW alone is probably 40 miles. Times 200 airports and airbases? You could triple fence the border it for less than those fences cost.
The only logistical impediment to a fence is the EPA.
And property rights as well as water rights, and international treaties...