Static objects regardless of camera in SceneKit

I am working on a game that involves piloting a ship through a long tunnel. I want to be able to have the tunnel the same size as the screen always so that the four edges represent the "tunnel". I'm using a SCNShapeNode to create each of the four "walls", but I can can't seem to figure out how to always have them pinned to the edges of the screen.

I've tried converting the screen size to the location on the scene, but it most often doesn't really work at all or at the very best only gets close.

Any ideas on how to make this work regardless of device would be greatly appreciated.

Thanks.