An artist’s rendition of what President Trump’s proposed wall would like, according to his stated specifications. (C. Huff for Yahoo News)

The ever-evolving vision for the wall President Trump wants to build along the border with Mexico got some new details earlier this month, the latest in the growing list of specifications for meeting Trump’s signature campaign promise — which is still in need of a source of funding.

“One of the things with the wall is you need transparency,” said Trump, referring to the physical construction, not the political process by which it is authorized, designed and paid for. “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.”

Trump’s announced reasoning — that without transparency, someone might accidentally get hit with a 60-pound bag of heroin thrown from the opposite side — came in for some ridicule, but the idea was baked into the U.S. Customs and Border Protection Service’s specifications months ago. “Incorporating a see-through component/capability to the wall that facilitates situational awareness but does not negate the requirements listed above is operationally advantageous” was the bureaucracy’s way of putting it in the request for proposals to design and build the wall. That bid solicitation served as an attempt to codify one of the central promises of the Trump campaign and administration.

“I will build a great wall ― and nobody builds walls better than me, believe me ―and I’ll build them very inexpensively,” said Trump on June 16, 2015, the day he announced his bid to run for president. “I will build a great, great wall on our southern border, and I will make Mexico pay for that wall. Mark my words.”

Republican presidential hopeful Donald Trump speaks to the media during a tour of the World Trade International Bridge at the U.S.-Mexico border in Laredo, Texas, July 23, 2015. (AP Photo/LM Otero)

The height of the wall — at least as described by Trump — has varied considerably, apparently in response to his mood. In December 2015, replying to a question from a child who asked what the wall would be built of, Trump suggested it might be 90 feet high.

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“I’ll tell you what it’s gonna be made of: It’s gonna be made of hardened concrete and it’s gonna be made out of rebar and steel,” said Trump. “You know, it’s so easy, that’s what I do. You ever see parking plank where they set plank and it goes 90 feet long? You know how long 90 feet … if you’re 90 feet up, you don’t wanna come down, you wanna come down very gently.”

Trump scaled back that height later in the campaign, saying in February 2016 that it would be 35 to 40 feet high. When former Mexican President Vicente Fox said his country wouldn’t pay for it, Trump responded by telling the crowd at a rally, “The wall just got 10 feet higher.” When he was asked about the comments at a February debate, he added another 10 feet to the total. Seemingly realizing how much of a winner that line was with his crowds, Trump added yet another 10 feet to the wall at an event where New Jersey Gov. Chris Christie endorsed his campaign, bringing the height to an estimated 65 feet.

Another important addition to Trump’s description of the wall was that of a “big, beautiful door,” which he brought up in an August 2015 primary debate after restating his claim that the Mexican government was sending criminals across the border. Those who entered the country legally, per Trump, could use his impressive door.

An artist’s rendition of what President Trump’s proposed wall would like, according to his stated specifications. (C. OB for Yahoo News)

The latest addition to the plans — and one not in the Customs posting — was suggested by Trump in June, first in a meeting with Republican lawmakers and then later at a rally in Iowa.

“You know, people don’t realize we’re already spending a lot of money on design, but I’ll give you an idea that nobody has heard about yet,” the president said.

“We’re thinking of something that’s unique,” said Trump of his wall at a June 21 rally in Iowa. “We’re talking about the southern border: lots of sun, lots of heat. We’re thinking about building the wall as a solar wall, so it creates energy and pays for itself. And this way, Mexico will have to pay much less money, and that’s good, right?”

“You’re the first group I’ve told that to,” continued the president. “A solar wall — it makes sense, let’s see, we’re working it out, we’ll see. Solar wall, panels, beautiful. I mean actually, think of it, the higher it goes, the more valuable it is.”

President Trump, accompanied by Vice President Mike Pence and others, taps on the table after signing an executive order for immigration actions to build border wall, Jan. 25, 2017, at the Homeland Security Department in Washington. (AP Photo/Pablo Martinez Monsivais)

While the official bid solicitation forms mention a “see-through component,” they make no mention of solar panels. They also envision somewhat lower height than Trump did, setting a nominal standard of 30 feet, and a minimum of 18. The bid also lists a number of other requirements:

It shall not be possible for a human to climb to the top of the wall or access the top of the wall from either side unassisted (e.g., via the use of a ladder, etc.)

The wall design shall include anti-climb topping features that prevent scaling using common and more sophisticated climbing aids (e.g., grappling hooks, handholds, etc.)

The wall shall prevent digging or tunneling below it for a minimum of 6 feet below the lowest adjacent grade.

The wall shall prevent/deter for a minimum of 1 hour the creation of a physical breach of the wall (e.g., punching through the wall) larger than 12 inches in diameter or square using sledgehammer, car jack, pick axe, chisel, battery operated impact tools, battery operated cutting tools, oxy/acetylene torch or other similar hand-held tools.

The north side of wall (i.e., U.S. facing side) shall be aesthetically pleasing in color, anti-climb texture, etc., to be consistent with the general surrounding environment. The manufacturing/construction process should facilitate changes in color and texture pursuant to site specific requirements.

The biggest question going forward on the wall is its source of funding. Despite Trump’s campaign promise, the government of Mexico has not stepped forward to pay for it. Rep. Mike Rogers, R-Ala., proposed a bill in March that would tax personal funds sent to Latin American countries as an attempt to pay for it, while Sen. Ted Cruz, R-Texas, has suggested taking assets from the drug lord El Chapo. The Trump White House’s budget proposal asked for $1.5 billion to start the wall, far below the estimated $40 billion one MIT study projected would be necessary.

Trump himself has begun to downgrade his initial claim, stating that the wall doesn’t need to cover the entire 2,000-mile-long border and could run as few as 700 to 900 miles. He also said earlier this month that the renovation of existing structures could count as part of his proposed wall.

“Plus we have some wall that’s already up that we’re already fixing,” said Trump in the same conversation with the press where he warned of the dangers of falling drugs. “You know, we’ve already started the wall because we’re fixing large portions of wall right now. We’re taking wall that was good but it’s in very bad shape, and we’re making it new. We’re fixing it. It’s already started. So we’ve actually, in the true sense — you know, there’s no reason to take it down. So in a true sense, we’ve already started the wall.”

Gabby Kaufman contributed to the reporting of this story.