The Trump administration has awarded contracts to build four border wall prototypes using non-concrete "other materials," U.S. Customs and Border Protection announced Thursday.

The contracts came a week after the administration announced four companies that will build prototypes of concrete border walls. In all, the eight prototype walls will cost about $3.6 million.

In July, President Trump Donald John TrumpUS reimposes UN sanctions on Iran amid increasing tensions Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally MORE floated the idea of constructing a transparent wall along the United States' southern border in order to prevent injuries from objects, particularly "large sacks of drugs," hitting people after people tossed them over the barrier.

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"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 told reporters. "As crazy as that sounds, you need transparency through that wall."

The proposed border wall was among Trump's highest profile campaign promises and has been the source of much controversy for the White House and congressional Republicans.

Within days of taking office, the president signed an executive order authorizing the construction of the wall and ramping up border enforcement measures, though action on the ground has been slow coming.

The four companies selected to build the non-concrete prototypes are Caddell Construction in Montgomery, Ala.; KWR Construction in Sierra Vista, Ariz.; ELTA North America Inc. in Annapolis Junction, Md.; and W. G. Yates & Sons Construction Company in Philadelphia, Miss.