I believe the character is sitting or squatting down in their square, and makes an illusion of a 5' section of the wall behind them. So just imagine like a cardboard cutout that looks exactly like the wall behind them and is positioned in front of them. Am I misunderstanding your question?
Ok, but even before that, I'm asking how can this really work well? I'm not seeing how you can effectively become invisible here. I feel like it would be obvious that a section of the wall is different/off.
Making a cube is even more strange. Now you have to choose, presumably, 5 points of reference to make sure the viewer coming from whichever side will be viewing you at the perfect angle? How?
EDIT:
Thank you for this lperkins, as it helps to illustrate the desired effect. However, I think there's an issue in that the Ghost Blind is an actual mirror, so the image it is reflecting will change with your perspective as you move.
An illusion won't do that. That's sort of the issue I'm having a problem with.
I'm not sure it's safe to assume that the wall is amorphous and that the floor looks the same. When I think of dungeon walls, I think of bricks or stones laid over one another. And when I think of dungeon floors, I don't think they look like bricks. More like pavers, or unworked, or flat.
Quick google search -->
dungeon wall
All those lines will do you in I think, unless the viewer is pretty far off and not really paying attention. I don't know...