You should aim for 50-75 watts of cfl's per square foot for flowering. And I would use the 5900k for veg and the 2900k for flowering. Using the 2900Ks encourages a higher flower to leaf ratio. Hang your lights horizontal if you can and make a reflector like you said except use flat white paint, aluminum foil on the glossy side only reflects around 30 to 50 % of the light causes heat problems while flat white paint reflects around 70-80% light and mylar is like 95%. I am making a small cabinet too 4x2x4ft for flowering and 2x2x3ft for veg. I'm using 4 of the 26w 5900ks for veg and 5 42w 2900k for flowering. When I get enough money to afford a carbon filter and proper ventilation I'm getting two 250W HPS and I'm gonna put the clfs around the sides. I found a bathroom light strip that goes above the mirror that has 5 light sockets and I mounted it sideways in a white box. It works great for a DIY light. The light it puts out is even and fairly bright and it doesn't make very much heat at all.