I have two of these, very nicely made, and very happy. Just don't polish the reflective surface too hard...
http://www.candlepowerforums.com/vb/showthread.php?t=163470
They are now available in different opening sizes. For the ROP, you need the 8.38 mm version. Then it's just a case of picking the texturing required.
I built a ROP 6D first. D-cells have very desirable discharge curves under load.
However, it did not see a lot of use due to its size (you look very suspicious carrying a lump of aluminium that big). The great news is that the parts were not a wasted investment because I could swap them into a 2D host, no problem. All I needed was a 6AA>2D adapter from FiveMega, and of course AA cells. LuxLuthor even does fully assembled 7.2 volt battery packs for the 2D.