I modified the block dimensions so the smallest blocks are cubic. I also only made four levels of blocks so that the blocks could be stacked into two levels with just eight cubes' worth of empty space. This simplifies storage, and works with the lid I made out of 1x4s to complement the tray that's in Ana's plans. The tray and the lid fasten together with window sash hardware.
I spent forever on this project because it was my first and I was trying to make it perfect. Watching my little one play with it, I now appreciate how soft pine is and how fast kids distress it!
I experimented with woodburning on the lid, otherwise it and the tray just have beeswax and coconut oil on them.