I have tried that with those plastic freezer packs, that works for as long as the pack is still cooler than ambient temps, and mine had to be changed out sometimes more than twice a day. It can work but dissipating the heat from the tent out of your accommodation would be a part of the whole solution.
Personally I would very seriously consider what MC said by dropping the plant number. Better to have 3 plants that do well than 9 that are average, cost you electricity and lose sleep.
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