You can put the fermenter in a cooler full of cold water up to the level of the wort, then just add frozen water bottles to the cooler to maintain the correct ferm temp, you swap out fresh frozen bottles for the thawed ones and refreeze them. also putting a shirt over the fermenter to wick water up to the top of the carboy works great as well, put a small fan pointing at the wet shirt and you have a decent evaporative cooler going.
So using this method, will I have to change out frozen water bottles for 3 weeks during complete primary fermentation?