Could be a fence or, could be a wall but with a fence in some places, maybe a stone wall or perhaps a lego wall with their "beautiful terrific very strong, very strong interlocking pieces", or some twigs leaned against each other, "the border has tremendous twigs, some are racist and some, I assume, are good twigs", or it could be bubble wrap, "I mean no one wants to hear all those bubbles popping when you walk over it", or an app, a wall app that shows where a wall would be if we built one so any Mexican using their cell phone would see the phantom wall and give up, or "you know I've talked to many people, many people, who say a wall in their heads is good enough, a tremendous mental wall that keeps people out of their imaginary conservative safe-space".