U.S. Customs and Border Protection is asking for design proposals and prototypes of President Trump's proposed wall on the U.S.-Mexico border.

Late Friday, the agency released specifics for the first time on how tough the barrier must be. CBP posted online two different options for contractors: one proposal must be for a solid concrete wall, another is for a wall with "a see-through component/capability" that is "operationally advantageous."

"The wall design shall be physically imposing in height," the CBP outlines say. The government says its "nominal concept" is for a 30-foot-high wall, but adds that designs as low as 18 feet "may be acceptable."

The proposal document asks contractors for 30-foot-long prototypes and mock-ups of 10 feet by 10 feet.

A CBP official told NPR's John Burnett that contractors will have to make mock-ups of their ideas in San Diego. "It's a way for the agency to identify designs. We're looking for industry's designs, to take a fresh look at the wall. We'll have industry propose and then we'll down-select."

The CBP, which is part of the Department of Homeland Security, requires the wall designs to be impossible for humans to climb without a ladder. It wants proposals that will prevent people from tunneling underneath by at least 6 feet underground.

The government outlines the types of things both types of walls have to stand up to: "sledgehammer, car jack, pick axe, chisel, battery operated impact tools, battery operated cutting tools, Oxy/acetylene torch or other similar hand-held tools."

It has to look good, too. "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 general surrounding environment," the CBP says. There's no mention of the aesthetics on the Mexican side.

More than 400 companies have told the Department of Homeland Security they're interested in the project, NPR's Richard Gonzales reported last week. Cost estimates of the wall vary widely: President Trump said it would cost $12 billion; an MIT study said $38 billion.

The deadline for contractors to submit their proposals for the first phase is March 29.

NPR's John Burnett contributed to this report.

Copyright 2021 NPR. To see more, visit https://www.npr.org.