This is the best way that I have found to make a 2-rank, 12-cup Mancala board out of egg-boxes. I have experimented with different methods and game pieces. There are other websites with similar instructions but most results are not quite so satisfactory or straightforward.
Now, you can decorate the game, if you wish with poster paint or maybe glue and glitter - I will leave it to your creative imagination.
The board will fold in half for storage. The trays either end can be taped to the ends of the game but it is probably better to keep them separate. For versions of Mancala where the trays are not part of the game play, such as Oware, you may prefer to put the trays in front of each player rather than at the ends. When being stored, one can be put inside the other.