Heat box help!

I know it is still summer, but I have lost my winter firmenting area due to the expansion of our family. I would like to build an insulated box but I am not quite sure what to do for heat and how to controll it. I don't have a a lot of space for it but I am going to make it big enough to hold 3 carbouys. Any sugestion!

Thanks

http://www.pets-warehouse.com/Vpasp/shopdisplayproducts.asp?id=2134

I use one of these in a 16 cubic foot chest freezer coupled to a digital controller (Ranco). As a test, I wanted to see how warm it could get the box and it reached a balmy 88*F at it's top end in that big box.

The digital controller isn't the only option but definitely make it hands free. There are mechanical versiaons available too for less.

You could also get a "Brew Belt Kit" that will include a controller and a heating pad that you strap directly to your fermenter.