When you make jello, the boiling water lets the gelatin form bonds, which trap the water in the little cells because liquid water likes to glom onto it's neighbors and it makes it hard for it to get out. Hence it wiggles instead of seeping water or splashing.

If you heat it, the gelatin bonds break and it goes to liquid.

If you leave it at a low temperature with good ventilation, the water molecules will randomly break their loose bond with their neighbors and fly out of the gelatin cell they're in. As this happens, the gelatin will contract (it "wants" to be about the size of the powder in the box, but it's caught squishing water) and get rigid, since the gelatin protein is naturally pretty hard (think bone or cartilage).
If the gelatin is too thick, the outer layers will get thin and rigid before the inner, and things will crack. It'll happen to Jello in the fridge after a bit if you leave it uncovered, which is why you put some plastic wrap on it.

All that to say, get your jello about a quarter of an inch thick and leave it in front of the air conditioning vent under your bed, then file it down on the roughest surface in your room.

https://youtu.be/Oadf4KNYz-I?si=ZusaL2wHNAdAaQCo

This person starts with a tougher gelatin, but it's the same principle.