I don't expect anyone to make the same box as I did, but maybe it will give some ideas working on your own project. 
   I've bought a 30 litre box and it was not rigid at all, but it was cheap for its size and had two "handles". So I made a spool holder that made the box solid after I glued it with silicone to the bottom. After installing the spool holder, the lid now closes flush. I wanted to make a gasket out of silicone, but there's no need for that with this approach. I wanted a box that had no screws in it, so I designed simple 10mm pneumatic fitting 25 degree brackets and joined them with silicone, same with hygrometer frame. I did testing over two months. At first no connections had silicone, so after one and a half weeks, humidity started rising in the box and silica beads changed color. I siliconed every connection and now almost two months 10% humidity with small bag of silica. One spool is feeded to the printer for a week now as well. I also did some “hinges” for the box. Actually at first I want to make the “hinges” so the lid would stay shut, but when I installed two one one side they serve as hinges and hold the lid when it's open ;D That was unexpected, but I liked it and left it like that because no air goes in anyway.
Here are some stl's :
https://www.printables.com/model/1043692-a-serious-spool-holder
https://www.printables.com/model/1054083-screw-on-funnel-for-500g-silica-bead-container
https://www.printables.com/model/1059660-frame-and-bracket-for-hygrometer

Also added some model files. Good luck with your project.

APPLY PAINTERS TAPE FROM BOTH SIDES BEFORE CUTTING THE BOX ;) If the box is very rigid it might crack even with painters tape.