Oh, I just remembered this video I watched. There is this guy on youtube that does a lot of material-sciency kind of stuff using common household ingredients. One of his videos he makes a salt-based phase change material, which basically works as an icepack that freezes at around 18 degrees Celsius.

The idea is that it takes a long time for the material to melt from 18 degrees to 19 degrees so it can keep you comfortably cool if you put it in a plastic bag and wear it in a vest to keep yourself cool. If you have a cool basement, then it can pretty much re-freeze by putting it on the cold floor.

DIY Supermaterial Could Save You From Heatstroke: Salt based PCMs

Its an interesting watch.

Edit: Actually the more I think about it the more Solarpunk it seems. It requires zero electricity, and rather than cooling a whole area you only cool the people in it. Even better, you aren't trapped inside to avoid the heat, as it works outside just fine.