Monkey - Congrats, one of the very few projects here at XS, that was actually designed from a true heat transfer point of view. For a week or so, I was doing a worklog of my most recent build when a poster asked me about passive heat flow and radiance. I didnt want to besiege him with math, so I had intended to answer that is a column standard radiator form (what you have), you need something like 28,400 grams of thin copper to conduct a roughly 500 watt radiance and stay at 50 deg C. (of course, as you read from the heat transfer text, 3D configuration is utmost in passive radiance)

Now I wish I would have had your pictures to show instead , LOL. Much simpler.