“Use non-union labor.”
Lots of ways to reduce the labor costs. Offer non-violent convicts a chance to earn reductions in their sentences for volunteering their labor. Or, make it a CCC-type jobs program - earn some money and learn a trade. These are just a couple of suggestions. I’m sure there are a lot more. People who are out of work because they lost their jobs to illegals would probably jump at the chance to put a stop to this epidemic of off-the-books labor by helping build the wall.
Navy Seebees. They are drawing pay anyway.
I wish Pres. Trump would set up a fund that Americans can donate money for the wall. I would also like to volunteer my time for free to help in its construction.