On wall, Trump doubles down on solar, suggests 'transparency'

— -- President Donald Trump repromoted some of his ideas for his proposed southern border wall Wednesday, telling reporters that he's "not joking" about the notion of making it a "solar wall," and explaining that the barrier needs to incorporate a level of physical "transparency."

Trump's comments on the wall, perhaps his most promulgated campaign pledge ranging back to day one of his candidacy, came as he fielded questions from reporters aboard Air Force One on his trip to France Wednesday.

Asked if he was "joking about solar," a suggestion he made in reference to the wall during a June campaign-style rally in Iowa -- and was included in the plans of at least one contractor submitting plans for the blockade -- Trump said there was "a chance" it could be done and that "major companies" are looking at it.

"Look, there's no better place for solar than the Mexico border -- the southern border," said Trump, according to a White House transcript. "And there is a very good chance we can do a solar wall, which would actually look good. But there is a very good chance we could do a solar wall."

During the June rally, the president said that adding the electricity-generating panels to the wall could "create energy and pay for itself," but during last week's G-20 summit, in which he met with Mexican President Enrique Pena Nieto, Trump said he "absolutely" still wanted the U.S.'s southern neighbor to pay for its construction.

Trump repeatedly pledged during the campaign that Mexico would foot the wall's bill, but has since acknowledged the U.S. could pay upfront and "be paid back by Mexico later." Treasury Secretary Steve Mnuchin told ABC News Sunday that determining how to pay for the project was "not the focus for the moment."

The president further detailed Wednesday that the wall would necessitate materials or a design that would allow for persons to have the ability to see through to the other side. Trump explained the requirement by providing an example in which smugglers attempt to move drugs over a theoretical partition.

"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," said Trump. "As crazy as that sounds, you need transparency through that wall."

In March, U.S. Customs and Border Protection solicited proposals for two types of walls -- one concrete barrier and another constructed of "alternative materials." Requirements included that the wall be at least 18-feet-high, but preferably 30-feet-high, prevent "scaling" and "tunneling below it" and be "aesthetically pleasing" on the U.S. side.

In June, CBP said it was entering "phase two" of the process and would construct up to eight prototypes to review.

In his answer aboard Air Force One, Trump additionally repeated a belief that the wall would not need to span the entire, nearly 2,000 mile southern border -- a claim he made as early as February 2016, prior to clinching the Republican presidential nomination.

"You don't need 2,000 miles of wall because you have a lot of natural barriers," he said Wednesday. "You have mountains. You have some rivers that are violent and vicious. You have some areas that are so far away that you don't really have people crossing. So you don't need that. But you'll need anywhere from 700 to 900 miles."

Almost 700 miles of the border currently feature some form of fencing, a result of the 2006 Secure Fence Act.

ABC News' Justin Fishel and Geneva Sands contributed to this report.