Mind explaining your criteria for "massive round-ups," so as to not violate the 4th Amendment?
Big wall. From San Diego to the Rio Grande - watch towers - military mans it.
(If the Chinese could build one 3,000 years ago, we can do it today, so I don't want to hear it).
China was ruled by a despotic government 3,000 years ago, and what the Emperor wanted, the Emperor got, no arguments accepted. The US is not in a similar condition. The wall may be physically achievable, but it is not fiscally achievable.
And the Great Wall ultimately failed in its mission.