I use the ranco two-stage controller. In my case, I have freezers, but I put stage 1 on cooling, and set the max temp and range for the freezer to cool, then I set stage 2 to the lower temp and range and it powers a silver dome light fixture with a ceramic heat bulb (no light to light-strike the beers).
I had to put a foil sheet hanging around the lip of the dome fixture to keep the heat from being too intense on the upper shelf carboy next to it, but otherwise this works beautifully and my fermentation control in the one freezer is great. My other is the chest freezer I age beer in kegs in, and it's great too.
Maybe ceramic bulbs would work for you?
Is your temperature out there steady enough that you can tell when you need to switch the controller from heat to cooling? Mine is very unstable in my uninsulated garage and North Carolina weather. In fall and spring I don't know when it will be too cold or too warm, sometimes both in the same day, so the two-stage is a must for me.