If Trump shut down the border, refused any trade Mexico would beg to pay for it. They can spend 5 billion or lose 50+ billion. Seems simple enough. Truth is, even if Mexico wrote a check the Dems wouldn’t appropriate it.
How about Trump make a secret deal with Mexico & get CONgress to give Mexico an additional amount of money for boarder in south Mex. Then have to Mex pres to not only build a wall on it's southern border but also on it's northern. IMO this seems too easy to solve ergo it will never happen.