you going to need more room prolly.

i have about the same size box. However i useing different lights. 150watt hps, and 85 watt cfl (true watts) .

My cab is made out of cardboard as well. Figures it beats buying a 200$ storage cabient. for venting i use dual 4" inline fans, don't know the cfm but its pretty high.

i would suggest building a second cab for when you have to upgrade cab size. I have mine just chillen waiting for it. At least i HOPE i need it :rasta:

as far as your problems, stick your finger in the soil, if its wet, stop watering your plants. moist stop watering them, for at least 2-3 days. if it is BONE dry then add just a tiny bit of water.

first plant looks like it was over watered. the second and third picture looks like either nute burn, or heat stress.