How should approach my second phase for flowering?

Please see attached pic of my loft! I am really close to moving my plants from cfl to 440 w sodium for 12/12 flowering. As I am making such a lot of schoolboy errors I wondered if anyone had any ideas about how best to utilise this space, bearing in mind ventilation, light contamination during the dark 12 hours and overall technique. I have a roll of black tarporleen (plastic) that I was going to cover the front of the space in a bid to make it light proof?

Any ideas ???