The US-Mexico wall should be 30 feet (9 meters) high, difficult to climb or cut through and look good from the north side, according to US government contract notices.
Two calls for tender were posted by Customs and Border Protection, the Homeland Security Department agency that will oversee the project and eventually patrol and maintain the wall.
The first notice asks for a solid concrete wall, while the other asks for proposals for a "see-through" structure.
Both require the wall to be "physically imposing in height," ideally 30-foot high – but potentially just 18 feet. Neither should be scalable, even using sophisticated climbing aids, and both should be sunk at least six feet into the ground to avoid tunneling.
They would have to withstand prolonged attacks with "sledgehammer, car jack, pick axe, chisel, battery operated impact tools, battery operated cutting tools, Oxy/acetylene torch or other similar hand-held tools."
The walls would include 25- and 50-foot automated gates for pedestrians and vehicles.
The wall is deeply reviled in Mexico with lawmakers calling for sanctions on Mexican firms involved
'Pleasing in color' – on the US side
Both should be "aesthetically pleasing in color" and texture consistent with the general environment, but only from the north side.
Successful applicants would be asked to build a 30-foot-wide sample wall in San Diego for final selection.
Trump's new budget proposal included a US$2.6 billion down payment for the wall but the total cost for the project was still unclear.
The Government Accountability Office estimated it would cost about US$6.5 million per mile (€3.8 million per kilometer) for a pedestrian-proof fence and about US$1.8 million a mile for a vehicle barrier. Congressional Republicans said the wall would cost between US$12 billion and US$15 billion and Trump suggested US$12 billion.
An unnamed government official involved in the project told The Associated Press that an internal report estimated the cost of building a wall along the entire border at about US$21 billion.
One of the proposals calls for a see-through component that "facilitates situational awareness but does not detract from its function
Mexican firms pull out
Earlier this week Mexican lawmakers increased pressure on Mexican construction firms tempted to help build deeply reviled wall.
Senator Gabriela Cuevas of the opposition National Action Party was due to introduce a bill barring the government from doing business with any Mexican firm that participates in its construction.
Mexican cement giant Cemex back-flipped on comments that it was ready to supply materials for the wall. Its stocks had risen to eight-year highs on speculation that the wall would require US$1 billion of concrete.
Another construction giant, Elementia – partly owned by Mexican billionaire and Trump critic Carlos Slim – said it wants nothing to do with the project.
A company in the central Mexican state of Puebla called Ecovelocity pulled back after initially saying it wanted to install lighting for the wall.
Trump's insistence that Mexico pay for the wall, together with his attacks on Mexican immigrants as "criminals" and "rapists," caused a diplomatic row between the two neighbors with Mexican President Enrique Pena Nieto canceling a White House visit in January.