wouldn't it be much easier to use a closet or to build a wooden box or to get a tent or to improvise one or to use an old fridge or to use an old computer tower case(lots of safer options).
Cardboard...not a great combination (heat, wires, cardboard, water, high humidity...all hazards)...it can work, but it's extreme (?!)
Best of luck with the grow! be safe!
Consider participating in a contest or so and maybe @GrowDiaries might have a nice tent for you to win or any of the big companies around to sponsor you!
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