The pics look good. I will post some on a build thread when I am up and going. Industry standard for fins, copper or aluminum, is between .3 and .4mm and 150mm wide between tubes. The manufacturers base the thickness and spacing on the time it takes the heat to move from the fin edge to the tube and, as pointed out, aluminum does not have the conduction of copper so an aluminum fin will be closer to .4mm and copper closer to .3mm for roughly the same heat transfer.
I have a roll of Bluetec absorber (good for about 28 panels) from Germany for an ultrasonic welder I bought years ago. It is still sitting on the skid. At the time, it was the most efficient absorber on the market and I don't think there has been much progress since. It is hard to beat 96% absorption and 5% emission.
If you have more panels to build, I would recommend buying a can of Solkote II made in New jersey. It has a very high absorption at about 95% and emissivity of around 20% but the main thing the higher emissivity does is limit the stagnation temp which is not that useful.
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