Put a portal in empty space, rotate the plane of the exit 90 degrees, then send a box through at a simple constant speed. Assuming the universe isn’t compensating in other ways*, the total momentum of the universe will be different before and after the box went through, so there’s no environment encompassing the portal interactions that will satisfy the zero condition.
Pretty sure the momentum from the perspective of the box stays the same. The game doesn't explicitly say that momentum for the entire system stays the same, just that "Momentum, a function of mass and velocity, is conserved between portals.” so rotating the exit of the portal in a different direction changes the momentum of the entire system (room, portals, box, etc..), but the box never actually experiences a change in momentum. Maybe a better way to put it, is that the absolute momentum of the box never changes when it passes through a portal.
