Congratulations on finishing the 250 boxes!

All in all I think you've done a pretty good job. Your lines and convergences clearly improve through the challenge. There are still some issues I want to point out.

You still have some problems with clear divergences on some boxes, such as 240, remember, that, as I show here, 2 lines won't ever diverge, they will always converge. So make sure that this doesn't happen, at least in your initial lines, as you can fix it just with thinking.

I also see that, even though it has improved, you still have some problems with the inner corners, 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.

If you want to know about why this works, it's 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.

I'm also glad to see you've drawn few boxes per page, and quite big. I recommend drawing them even bigger. Don't worry if the lines of the extensions collide with other boxes, it's not important.