Found some info.
Idea for a thermostat -
http://hackaday.com/2009/03/22/brew-fridge-thermostat/
A problem that could arise is fan noise if I use one. Just a thought... I think a metal plate for a heatsink spread across the back would maybe heat the back of the backpack! lol
Do you think the thermostat will work? Or do you think the whole project will work?