I'm not really sure I understand the setup you're describing, so I can't be of much help in evaluating it.
However, I was interested to hear about your good fortune when it comes to the Dry Ice, and I have other ideas about its use.
Since Dry Ice is a temporary solution, it would probably be best to employ it in some fashion that would allow you to get the most time-use out of it. Unless you're planning on using some sort of complex water-to-ice intercooler to keep the Dry Ice solid longer, I think the best way to use it would be in an air chiller setup.
You could place the Dry Ice in a heavily insulated container with openings at both ends (a drilled mini-cooler would work). Then, run tubing (also insulated) into your case and connect it to a heatsink from the side in a straight-through configuration so that the chilled air is exhausted back out of the case (gotta keep that condensation down). This would be very cold air acting on the heatsink, and, since there is no direct heat source acting on the Dry Ice, it would remain solid much longer.
Just a thought.
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