I store the trade cards in the trays (works fine if you keep the box level.)
I took a cardboard box and trimmed it to fit the advancement cards
I used your idea and stored the trade cards in the trays. I didn't think they would stay, but they actually stack reasonably well that way.
I found a perfect sized box at 'The Container Store' at a local mall here that a friend told me about. It's about the perfect size for the advance cards.