I've got an X-Wind fan to defog the lens, plus I have another fan to keep my face cool on those really hot days. The second fan runs off of two 9v batteries in a belt mounted box. The batteries made the mask too heavy to be comfortable. If I have them both on at the same time it gets pretty noisy.
Here's the switch and the connectors for the battery box. (The gunk on the switch is dried contact cement from trying to attach the rubber switch cover)
Here's the battery box itself.