Now, draw two vertical lines, straight up, from where that back line meets the two vanishing lines, up to the two top vanishing lines. Then add the horizontal line that joins them.
The two biggest problems at this stage of the drawing are lines at angles - they must be straight - and lines that don't quite meet. If you stop short or go past the vanishing line ever so slightly, with one of the lines, you'll have trouble getting your last line straight.
If your box is close to the horizon or vanishing point, you might find that the angles are very obtuse (wide), and hard to get right.
