First of all congratulations on completing the box challenge! Both of your lines and convergences improve, good job!

I'll point out a few things so you can keep improving.

-First thing I notice is that your lines have a bit of wobble:

In these exercises, you should always prioritize confidence over accuracy.

A wobbly line will always be worse than a confident line, no matter how off the confident line is.

If you take a look over the ghosted lines notes you'll see the levels of lines:

Level 1: Line is smooth and consistent without any visible wobbling, but doesn't quite pass through A or B, due to not following the right trajectory. It's a straight shot, but misses the mark a bit.

Level 2: Line is straight, smooth and consistent without any wobbling and maintains the correct trajectory. It does however either fall short or overshoot one or both points.

Level 3: Line is straight, smooth, consistent without any wobbling. It also starts right at one point and ends exactly at the other.

As you can see, wobbly lines aren't mentioned, which means that they would be worse than level 1.

-Second is inner corners, they actually end up being pretty good, but here is a demo to show another method to approach drawing boxes. Instead of completing first the outline of the box, you draw the inner corner before, as it's shown, in the fourth step. This helped me in figuring out the inner corner issue and hopefully it helps to improving.

-This helps because of this. Thinking about the relationships between lines will lead to better boxes. Drawing the boxes in that order will help you think about this when drawing the lines more, which is why it usually leads to better convergences. The method I posted earlier makes you it easier to think about the relationship of some lines, which is why I think it's better than drawing first the outline and lastly the inner corner.