Excellent improvement over the course of the 250 boxes! I'm working on that now.

One thing I've started doing to align stuff (which you might already be doing) is to use your pen to eyeball angles. I hold it right on the page and use it as a visual straightedge, sliding it around to estimate convergences of future lines. Then I pick it up and do the line.

On the back corner, I try to figure out how the longest line will align first, then find the point along it that works best for the two shorter ones. Then I place a dot and check all three lines before drawing them. (Sometimes it takes a few dots.)