I think that instead of asking for a wall, Trump should instead be focusing on, oh I don't know...
Connecting the more than 100 already-existing mini-walls along the US-Mexico border?
I mean yeah, there's the really big walls near the official border crossings, but then there's also the ones that regular people, or the border patrol, or homeowners living near the border have put up as well. Walls that stretch for a good 10-30 feet across and then just... stop. And then half a mile in either direction, you see some other wall/fence set up, sometimes closer/further from the border than the wall you just saw. And there's a good 300 foot gap between the last wall and this one, so it's not really doing its job.
Also there's the 'wall' in New Mexico that's literally just a 3 foot tall sheep fence going for about a mile or two, so there's that. Texas has a few large sheets of metal sticking out of the ground in some places, which I think are supposed to be the walls, just not attached to other walls/poles. Why not just try to connect the already-existing walls and demolish the less-than-effective ones?