My goal in designing game inserts is speeding up setup/teardown and avoiding annoying plastic bags whenever possible! There's just three parts needed: 

1. Top layer to hold reference + cave cards, scorepad and the Forest board; 
2. Card tray that fits everything in two stacks, leaving the third slot as an optional discard during play;
3. Spacer which fits around the card box and holds up the top layer.

Usually I try to avoid “wasted” parts used just to fill space, but this game is incredibly simple and just doesn't need more than two functional pieces. As a compromise, the current version of the spacer is as minimal as possible while still also serving it's structural purpose. It doesn't need to be removed from the box during play, so the card box can take up a bit less table space this way.