Yes cardboard is flammable but so is a tent or wood etc... I keep all wiring on brick walls and suspended in the air. I use cable covers when needed even caulked it in cases to prevent O2 from entering the tube. the cardboard is mainly the wall which reflect the light from the sides.I personally prefer a box because of the solid walls and given my choice, I prefer that for my grows. Hanging all the stuff you need can be somewhat problematic in a tent. However if you are not handy, like to move your grow or take your grow down, a tent is the way to go. If you have access to scrap lumber and have tools you can build something that is exactly like you want. I also feel a little bit of trepidation using something like cardboard with a high powered HID light.
I use the tents with the waterproof lining so that when my hydro system overflows and fills the bottom of the tent in 6" of water, it submerges all of the wiring and connections to keep them cool while everything is on :doh: