Update - the day after this thread started, I cryogenically pre cooled my coleman extreme to -76F (any cooler would have made the structure brittle and risked failure) then put some inside out frozen slo melt ice in. I filled it to the top and haven't opened it since, I am happy with the results (1 cube left after a month, not bad). The only problem is I have a rope handle on one side and busted hinges.
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