During WW2, the US built the Alaska highway in a single season, starting in March of 1942. The highway was dedicated in November of 1942. The total distance was 1,202 miles. Two crews worked toward each other and met at mile marker 558.
So, don’t tell me we cannot build a suitable barrier along the Mexican border. The Alaska highway was built using 10,000 men. We now have over 90 million unemployed adults in the country.
Begs the question of how many of them really want to work; especially if it's hard work. Idleness does not tend to make one work-ready. That said, there will be enough workers for such a project.