He has said that the wall would be built from precast concrete and steel and that it could be 50 feet tall, if not higher. After calling for it to extend across the entire 2,000-mile southern border, he more recently said half that length could be sufficient because of natural barriers. He has pegged the cost at $4 billion to $12 billion, most recently settling on around $10b. Some see that as low. “There’s a lot of logistics involved in this, and I don’t know how thoroughly they’ve thought it out,” said Todd Sternfeld, chief executive of Superior Concrete, a...