About the box
I thought back to both the game and me playing the game.
As you suggest it is necessary that all the pieces are visible at the same time, this way the player can better understand how to place the pieces together (which is useful with the more difficult puzzle).
This led me to the final conclusion of having all the pieces in a bigger grid (as also suggested by a user on reddit), the central part of the grid becomes where the pieces go to complete the puzzle.
This removes the need for a box.
At the moment I want to nail the main game mechanic, so I am leaving the polish for later.
I appreciate you playing my game and take the time to write me your opinion, thanks!