Trump also scaled back significantly the number of miles he predicts such a wall will span, to at most half of the southern border.
Speaking with reporters on Air Force One on the way to Paris, Trump described what he still wants for his pledged border wall -- which thus far Congress has not appropriated any money to build.
Picking up on the theme laid out by Homeland Security Secretary John Kelly and other border experts and agents, Trump spoke about the need for any border barrier to allow agents to see the other side. Much of the current barriers along the border are metal fencing.
"One of the things with the wall is you need transparency," Trump said.
"You have to be able to see through it. In other words, if you can't see through that wall -- so it could be a steel wall with openings, but you have to have openings because you have to see what's on the other side of the wall."
Experts have noted that with potentially violent cartels looking to smuggle people, drugs and other illicit goods into the US, it's essential that agents have visibility on their movements.
Trump offered another example.
"As horrible as it sounds, when they throw the large sacks of drugs over, and if you have people on the other side of the wall, you don't see them -- they hit you on the head with 60 pounds of stuff? It's over," Trump said. "As crazy as that sounds, you need transparency through that wall."
Trump also acknowledged Thursday that "violent and vicious" rivers and natural barriers along the southern border likely mean that the wall will not cover the full stretch, which is nearly 2,000 miles.
"You'll need anywhere from 700 to 900 miles," Trump said, in a substantially scaled back vision than he once touted.
Given current pacing for the wall, it could take a decade and tens of billions of dollars to make his plan happen.
While Trump has long committed to building a wall and the Department of Homeland Security has supported the idea, the administration has not put out a complete plan for what they envision. Congress did not fund a request for roughly $1 billion to start with roughly 60 miles in 2017 and a potential shutdown
fight is emerging over a $1.6 billion request to build just over 70 miles in 2018.
The building of the wall will be an "iterative" process, officials have said repeatedly, where the agency makes requests each year.