The dreaded light reflector debate.
For one thing the light you see is not the light the plant sees or needs.
Unless it is perfect and very expensive at that, a flat white surface is far better than foil or a mirror to reflect the light the plant sees and needs.
I used a towel on top of my frame and white paper plats for reflectors on my 12 CFL's I used.
I put painted white card board on the sides.
This worked by far better than anything I have ever used before.
The plants didn't even care too much how far away the light was.
With 1200 watts of light in that small of a space you simply cant go wrong.
Worth