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A removable back plate would indeed solve most of the watercooling issues with a air-only solution (i understand your air only preference, but with >60% humidity points here on my country, high density only happens with water blocks).
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Out of curiosity, what constitutes as high density? in kW per volume.
I've been doing some custom watercooling stuff for fun, wondering how thin the cooling blocks should be. So far I've only gone down to (in total) 7 mm thick blocks and they have had problems with flow.
As for liquid cooling it should generally require little (or no) servicing, especially since miners aren't kept running for long.