This is just a simple insert for the box since it is too nice to just toss but is not of much use otherwise. Easily fits single sleeved cards with your spellbook and atlas side by side, you would need to break up the spellbook a bit to fit double sleeved cards. Not a ton that can be done about that since the base of this is only 0.5mm thick so this is just a product of the box height.

You can use some double sided scotch tape or a few dabs of glue to hold it in if you like but it is not necessary.