Not sure if you decided on a method, but this is what I did for my heating temp control.
I connected a small 1500 watt fan heater to my Electrical Johnson control that is attached to my fermentation chamber (a True model Pepsi Fridge). I set the controller to heating mode and to cut-out at my set temp (this involves moving the pins inside the face of the controller, real easy)
The fan circulates the hot air throughout the fridge, cuts off right at my set point. Works like a charm.
I set the probe of the temp control right against my carboy, covered it with insulated bubble wrap and taped it down.
Here is the link to show you how to set the pins of the Johnson controller to heat mode:
Hope this helps.