Thank you for making the game intuitive, bite-sized, and clever! So many game jam submissions either have too complicated of mechanics, have far too much playtime, or just don't do anything special. Also thank you for doing levels, wish everyone would do levels!
What shines here is definitely the main mechanic or joining together with a box and the clever level designs that focus on that central idea. You get huge points for originality and design in my book, I doubt anyone else in this jam will think of such an idea and come up with such interesting implementations.
I think you should continue working on this after the jam is over, so here are some things I think you could improve in the future, otherwise great job!
Your game comes across to me as a puzzle platformer especially with the central mechanic and I think you do a great job at that part of it. However, I would say there are times the game starts to dip into precision platformer territory, where you require the player to time their movement or their jump a bit too much. I like the variety, however, I think the timing window was too tight sometimes and I think that adds unnecessary frustration and detracts from the brilliant puzzle designs you have. I wouldn't take those out, just make them 50% easier so that you keep the variety but don't make precision platforming a focus.
The art style and colors make it stand out, however I would still say you could have more art, particles, animations, etc. that still fit within the same style. Again of course with limited time and limited manpower this is totally understandable. Also, I think the game would benefit from a little music and sound for things like jumping, landing, joining to a box, shooting the projectile, projectile hitting the box, projectile hitting the player, player dying, player respawning, etc. I think you are allowed to use free music and sounds if you have legal right to use them, although of course original audio would be great too. It's hard for me to say whether things like camera shake or post processing effects would help with this style of game, but they are also worth considering just for polish.
Overall, really phenomenal submission and I think you will rank highly because of your original mechanic and designs!