Not having rear air vents sucks, but I've found that if you turn the center vents inward and level the air diverters that when you crank the a/c fan speed up you can get that air on the face feeling in the back seats if the passengers (my two kids) lean toward the middle.
The problem is that after a while, my wife is usually freezing in the front.
I've seen that 3D printed piece for under the seat too, but always figured it was more of a gimmick.