I should of rephrased that "there are things you can to improve your chances of not burning down your cardboard growbox" ... Of course it can burn down, houses burn down due to electrical failures all the time.
Reconsidering, I wouldn't ever make a cardboard box myself.. Too many complications and the risk of fire definatly is greater, if something goes wrong with your electrics [for whatever reason] you want the most flame resistant material around.. In some cases it won't matter if you have wood, plastic, cardboard.. Your shits burning down anyway. Some could argue not to make it out of wood either, but there are more complications than just being flammable..
I will agree, do it right the first time..
--SS