Great job on completing the box challenge! It really is very hard to manage finishing it, not everyone can do it! You've definitely improved through the challenge, specially on your lines.

I've a few things to point out so you can keep improving in your boxes:

First one is line extensions. You should always apply the extensions away from the center of your initial Y, as you can see here. On some boxes like 231 you still have this problem, so use this trick not to make it anymore!

For the convergences, you still have some that diverge on first stages, like 239. This is something that can never happen, as I show here

For the inner corner placement, you've improved a lot. I made this 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 you too.

All in all pretty good job. One thing that would have helped is drawing bigger. I actually recommend drawing 2-5 boxes per page max. Draw bigger boxes next times you practice them, it really helps!