Hi there! Grats on finishing the box challenge! I can see a great deal of improvement over the set.

One of the issues I see is that your lines are a tad wobbly at times, don't forget that in these drawabox exericses a wobbly line will always be worse than a confident line, no matter how accurate the wobbly line is, so don't forget to always draw confidently.

Another is the inner corner, which you're having trouble with. It's completely normal, but check this order of drawing boxes, and give a look at this diagram. Thinking about the relationship of all lines instead of thinking on pairs will help a lot on making the guesses of the lines better, inner corner included. Give it a read.

Overall like I said pretty good job. Just as a last thing, don't be afraid of filling the page with big boxes, it doesn't matter if the convergence lines touch other boxes.