Venting for a single exhaust Portable AC
I had this issue in an old office of mine which which was a room we subleased from an architect who apparently didn’t believe in air conditioning.
As you have observed, single pipe portable AC units create negative pressure in a room because they use the conditioned air from the room to cool the condensing coil which is then expelled from the room.
I identified the grill on the unit that was pulling in the conditioned air to cool the condensing coil and made a plenum out of cardboard which I taped to the unit, and ran a duct from this plenum to pull air  from outside the room, which balanced the pressure.
If you do this, make sure that your intake is below your exhaust so that you’re not pulling the hot exhaust back into the unit.
After the first day of using my modified 2 pipe system, the temperature of my office dropped 15°F!