hey i use a cardboard box too... do it! it works great!!
make sure you ventilate the air well to get the temps low (mine keep at a steady 78-80 degrees, ideally it would be slightly cooler but it's fine), line up the inside with emergency blankets from walmart and get to growing!!!....
i've left my box here for 5 days, with a timer, and no fire. mine is aabout 1.3 square feet, fits one, maybe two plants. if you want to use a bigger one go ahead. i've seen a couple of people using cardboard boxes with HID lighting and no issues like fire or anything, just don't overload the breaker with a bunch of stuff, just to be safe...
if you want to use a HPS or something like that make sure about the ventilation and you should be fine.
cardboard boxes tend to be short, like 3 feet, so if yu want to use two, one on top of the other do that... mine is 3 feet tall, and i "domed" the top part, that added an extra couple inches, PM me if you want to see what i mean.....
i personally love my cardboard box because i can have it in my bathroom (I leave the extractor that goes out the top of my building on to help with the stench, i use carbon sheets, haven't built a carbon filter yet) and move it when i have visitors over into the room and outta sight in about 1 min.... it's perfect for my purpose, small, lightweight, portable, and it works.
the problem is to completely seal the light out is kinda tricky, so tricky i even move the plant into the closet for the dark period just in case.....
so go ahead and use one!!!!!!good luck
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