How can I make my box lids less tight?

I’d try sanding the box lip, or the inside of the lid, or both.

If you have a shoulder plane you could take a few shavings off the lip. Paste wax the lip and the inside on the lid to reduce friction. You could also consider putting in some kind of a channel or something to relieve suction. I've got to believe that contributes to them being hard to open.

The lids for these boxes are too tight, and when there is enough humidity, they seal up because of the inner walls. How can I fix these boxes and what can I do for the next time that I make similar boxes?

Sand