Warning, this message is intended to stimulate an "out of the box", open minded brainstorming on securing the US- Mexico border, since ONLY the "wall solution " limitations and/or costs are being noted mostly to discredit the POTUS.
How about we pivot to a 25x25 yard trench, filled as needed with nuclear reactor spent fuel rods, so a 25 yard radius deadly event horizon results. Since Yucca mountain reneged as a waste disposal site, this would be better than what we are doing now, require no maintenance or surveillance except where the entry portals are. This plus a cyclone warning fence with a 1 mile dead zone using Roundup before reaching the trench may be all we need.
Expect LOL reaction, but reaching for a "this might work if we ....."