Simpler. The wall does have money budgeted for it, just not as much as Trump wants. Companies are going to be constructing sections of it.
Just go to a company, and make a deal with them. They get Kolfage’s money invested in them. In exchange, they offer to build X-yards more of the wall than their bid was originally planning to cover, with the extra being designated “The People’s Section” of the wall.
Outstanding thinking, PapaBear.