reflector surfaces

Hi all, I was just in the process of setting up an old 400W MH I have, and was curious, whats the best type of reflector to use? White? Aluminum? Polished? Hammered? I looked on ebay and there all kinds of different ones so I was just curious to get some feedback. Also I am thinking of making my own as a cost efficient option so in that case what should I use? Thanks in advance.

A pice of aluminum hard round furnace ducting (However big seems appropriate maybe 8" I'm guessing) is easily opened at the seam and bent into a batwing, painted FLAT WHITE with hi temp paint from an auto store...

I mean if you want to DIY it's pretty cheap and easy and works well.