That's it! The temperature controller is an STC-1000, which is inexpensive and can be sourced easily online.
Here's a quick pic-based wrapup of how we built a warmbox to keep kefir and sprouted brown rice warm during the winter.
We started with a steel rack so we didn't have to build a box. First, we removed the upper shelf and inverted it so we didn't have to work around the internal bolts when making our measurements. Next, we cut the upper and lower sheets of rigid foam insulation.