The thermal coefficient shouldn't depend upon either hot or cold-side temps.
BTW, Joe did have Xjinn's block at one point, and gave it back to him. Xjinn was going to send it to pHaestus for testing but never did.
As for machining time... I made one too and it took me 2 months of lunch hours to do it (most of the time was spend doing setup and cleanup for each session with the drill press). Small holes are a pain to drill if they are long, and even if it was aluminum rather than copper, I still broke three drill bits. Worked well enough, though. I was running it for about 2 years.
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