Don't use cardboard. It absorbs moisture and has no rigidity. If you can afford $10-$15 you could buy 2 large 32 gallon storage totes and flip one onto the other so both openings are facing each other creating a large dome. Line the 2 totes with the panda plastic, cut a few ventilation holes and add light, plants and nutrients and you have yourself a starter space until you can afford a larger cab/tent.