Either way I don't agree with you.
The first is a form of visualisation. Visualisation can work very well, Darrel Pace imagined a tunnel in the air leading to the gold, all he had to do was shoot the arrows down this tunnel and he couldn't miss. Obviously the tunnel doesn't exist, but if an archer can make himself believe that it does it can help the scores. I imagine shooting an arrow into the centre of the target, and then clear my mind, and try to just let my body do that. Sometimes it even works! I think that imagining a box could be counter productive to some people as it can keep things out just as easily as keeping them in. Still it's what works for you, and I'm not going to say that you're wrong, just that I wouldn't advise this method to any archer that I coached
If you are talking about picking a point to aim at and imagining it to be a box then I have less problems relating to your idea, but I would recommend that if you are going to shoot point of aim like this then try to find something to actually aim at, even if you focus on a blade of grass or a small piece of the target. Focus on that point and then bring the arrow into view until it touches or covers that spot (the choice is yours) and when it's there and you're at full draw then loose.
I do agree with what you've implied in that you shouldn't hold a
longbow at full draw.
Daniel