The bucket above the analogue board is not louvred, and that part of the case gets damned hot. Not warm, as CRT iMacs do, but hot.

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Sounds like if you use your Mac to read a document in the morning, you can

cook your eggs on it.

There is a definable stagnant 'well' of hot air in the top of the internal space that is not easily replaced.

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The hot air definitely needs to be drawn out.

I have long contemplated addition of active ventilation to my Mac.

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I agree.

Based upon the discussion here I am now of the opinion that active ventilation (or a chimney) must be used as a general practice. it should only be operated on its own for short periods.

Thanks for your valuable input.

--David