What are safe temperature ranges for fermenting miso paste?
According to http://www.soya.be/make-miso.php

"If temperature is too high the koji culture could become inactive."

"The container should be place in a clean room with moderate temperatures (15°C – 25°C)."

Like yeast I think the only danger is that if temperatures get too hot it will stop working or living. Your best bet at keeping it cool is likely putting it in a dark cupboard or somewhere dark with some kind of insulation ideally to stop large temperature fluctuations and stop it getting too hot. You get foil insulated picnic bags for example. Or newspaper or foam would also help insulate.