Very nicely done. Good mechanic with the box creation. I appreciated that there was a narrative. Also, you taught the player well, which is often lost in game jam games. I liked the graphics, too.
While the general movement and jumping felt good, I felt like the box creation felt a bit weird. I felt like it required too many clicks, and didn't really get a sense that my jump was reset when I created one -- probably due to the box instantly falling when it spawns. I also can't count how many times I jumped and instead of spawning a box I despawned the previous one, which was maddening. A few minor tweaks would clear all of these up.
Nice work!