Humidor Temperature

I live in Boston, and the weather is getting hot...finally! My wife and I live in the city, and keeping the temp in my Humidor low has been a bit of a challenge. I understand that the ideal spot for a Humidor is in a cool - room tempature area. However, does anyone have any ideas as to how to cool the inside of the humidor if that's not always possible? I noticed my unit is getting up to 72-75 degrees...too high. I thought about freezing distilled water and placing that inside the Humidor, but I'm not sure if that's a good idea...

Dirk