I wanted to share my foamcore insert for the core set box. Maybe some of you will find inspiration for your own (I'm always happy to see improvements and modifications).
It holds 3 decks in Ultrapro sleeves, plus all of the fate, honor, and honor/dishonor cards from 3 core sets. There's only room for 2 sets of rings, but I don't expect I'll ever need the third. It's not shown in the picture, but I put some pieces of foamcore underneath each deck to make it easier to lift them out.
If you decide to make your own, be sure to test fit the cards before gluing things in place. There's not a lot of extra room in the design, so larger sleeves may require adjustment. I know that the FFG sleeves are a little longer, and they don't quite fit in the space I have.