Commander-In-Chief, The Border & Emergency Powers

President Trump (And any POTUS), as "Commander-in-Chief" of our Armed Forces, has the authority to:
1. Declare parts of our border with Mexico as "Military Exclusion Zones" (Or such security sites as "Area 51") and restrict, with the use of lethal force allowed to maintain security, unauthorized crossings;
2. Provide for the "clearance" of residents and their guests and employees to be in such places;
3. Move our regular troops, reservists and National Guard (Military police, combat units and other personnel) to those places to prevent the illegal entry-into and passage-through those places; And,
4. To place those areas under military law (eg As civilians are on US Navy ships) with violations of the UCMJ to be by courts martial, with appeals to the "Military Court Of Appeals" before any appeal to SCOTUS.

It would be best to limit such actions to those wild areas as cannot be effectively controlled by the "Border Patrol"or other civil authorities. This would allow those non-military forces to be focused on: Airports; Seaports; The arrest, detention, prosecution and expulsion of those aliens who have overstayed or otherwise violated (eg Working and stealing jobs from Americans) the conditions of visas; More intense scrutiny at the usual places of entry.

I still maintain that all  "barriers" to illegal entry into my nation be supplemented by four-plus rolls of "razor ribbon" (aka "Barbed tape").

Funds for such actions could be had within DOD (eg By reducing "Flag Officers" to a much smaller portion of military personnel).