How about a glass jar with water and airstone / pump?

Using frozen water bottles will drop the local temperature and produce condensation which will keep everything moist.

Why is it that you want to keep the temperature down? is it for the dormancy period?

What plant(s) are you tying to maintain?

I'm keeping a Nepenthes Talangensis. I'm thinking that with the cooler temperatures of winter my bedroom might be cold enough to keep the tank cool, but I was just searching for any betters ways to maintain a consistant cold temperature.