There will be prototypes made by various companies. These prototypes will be broadcast by Faux et al as evidence of the wall being built. There may even be an award, or two or more companies will get the job (it's not Trump's money, so this will be extravagant. Trump will probably figure a way to tunnel money to one of his business).
Then the news cycle will proceed to some other item of distraction, and whenever the issue of the wall is raised the footage of the prototype construction will be brought out as evidence. The base will be firmly convinced the construction is proceeding.