Passive cooling works best when the radiant surface is facing up. So, a large radiator laid flat would be more efficient than a copper coil on the side. A 12x6" transmission cooler on the top of your case would probably work well, assuming you can get air to rise through it. A couple of low-speed, low-noise (Panaflo L1A's) 120mm fans pushing air through the radiator would help no-end, as long as there are case vents to let more air in.
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Melvyn
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