This is great concept and well-executed. Once you get the hang of movement the rest is simple and easy to learn, but there is a smooth learning curve to the existing puzzles, and room for a lot of challenging puzzles in the future.
The game itself is short, but there are instructions on how to create and submit new levels right in the project README; adding a "puzzle creation mode" to the game itself might be a way to encourage players to submit more levels. It also feels like it could be easy to extend the concept with new mechanics, like spaces only the player (not boxes) can enter, one-way movement spaces, etc. I would very much like to see more of this in the future.