I've been working on better temperature regulation. I took some measurements of the TMP36 I was using, and found that it had on the order of 10mv p-p of noise. This was the best case with lots of shielding and holding my breath. This corresponds to about 1 kelvin of noise in the temperature. I have decided that for the next revision, I will use an NTC thermistor in a bridge configuration. I have heard that these can be lower noise than the TMP36.

Here is an LTspice schematic of the temperature control system:

It's fairly easy if you break it into chunks.

On the left hand side is the virtual ground generator. This allows me to have a split supply for the opamps without actually having one. In the middle is the resistance bridge. Changes in temperature (and therefore resistance) will create a voltage differential between the two halves of the bridge. The Difference amplifier U2 will take these two voltages and create an error signal. The amplifier U3 forms a crude PI controller to servo the temperature. In the real deal the output of U3 would be connected to the heater transistor. Component values will probably change once I finalize parts.