I can’t figure out why they went with fire. Lightsabers are glowing rods ( toruses if you read the canon explanation) of glowing energy, not flaming swords. Fire would not provide an even glow. It would flicker. It would not be any of the proper colors without adding a lot coloring agent (eg copper for green flames). You’d get a look much closer to the films with some d cells, some wires, a bunch of light emitting diodes, and solder.
What I’m stuck on is not the stupidity, but how bad an imitation of light sabers they would have been even if they had worked. Hell, get a dimly lit room, and pick up a flashlight and an extendable future sword from the dollar store and you’d have a better looking lightsaber.