Hey FJ - if you still feel like giving that artificial horizon a shot; instead of using a large square'ish plane, I've had real good luck with using a (smaller) 'disc'.
With careful camera angles when the 'horizon' is in view, the curved edge helps sell the illusion. Additionally, it's possible that Navis' interprets/renders the disc elements differently than those of a poly, but that's just a shot in the dark.
HTH