Your right, the flat white will reflect (about 90% of given light), BUT light reflects at the opposite angle it hit at. Meaning if your light is going out in all directions,it's reflecting in all directions instead of downward towards your plant. I mean, you are in a small enough grow space that light is used pretty efficiently regardless, but without propper directionality, a lot of your light is used on wasted space.
I'd say you don't even need to make a shhet metal reflector. Just find something with a parabola shape, paint it white, and call it a day!