I think he could go to the Army Corps of Engineering and put them to the task. He may be on limited funding but it’s a start.
I'd say building the wall to provide security at our southern border falls into that category, namely, preventing the flood of illegals raging across our border.
The US Army Corps of Engineers is already managing the contracting for wall construction - they just need the money to make awards.
The Department of Homeland Security comes to the Corps of Engineers as a customer with money, and the Corps provides services like a general construction contractor.
The Corps of Engineers has two standing contracts that they have prepared to build segments of border barrier for DHS - an Eastern one (Texas) and a Western one (the rest).
Eastern Contract: W9126G18R0092 Multiple Award Task Order Contract (MATOC) for Horizontal Construction Services in support of the Rio Grande Valley, Laredo, Del Rio, Big Bend and El Paso Border Patrol Sectors
Western Contract: W9126G18R0096 Multiple Award Task Order Contract (MATOC) For Horizontal Construction Services In Support Of The San Diego, El Centro, Yuma, And Tucson Border Patrol Sectors
Developing those contracts was a big effort. All the requirements had to be specifically defined. The routes had to be specifically identified and surveyed, to identify property and other legal issues, as well as drainage and flooding issues. Construction companies had to be screened and pre-approved to compete on segments, to ensure that they had the financial means to perform, and that they had good records of past performance on similar construction projects.
All of that difficult planning and contracting lead time is now complete, and the Corps has been making awards with the money that DHS was appropriated in 2017 and 2018. Three awards were announced just last month - two in the Rio Grande Valley South of McAllen, Texas; and another around Yuma, Arizona.
The Department of Defense could just identify something to sacrifice out of their budget, and send the Army Corps of Engineers an email with the fund cite code (account number) for the money. The Corps could go ahead and use that to contract more wall segments.
The only thing needed now is money, where ever it legally comes from. The whole wall program is staffed and operating. They are ready to execute the funds, and have already demonstrated that they do it well, with the segments that they have already awarded.