From your darks...and from what I saw with cold boxes. Canon sensors below ~5C don't have appreciably less thermal noise. So probably aim for ~5C. Being that it takes time to cool to this....you have to make decide on a temp at some point.
If you want a simple controller....
WH7016R. yes its a simple thermostat but surprisingly I found it could
regulate sensor temp (I think its was sensor) within +/- 1 degree C (actually +/- .5C).
Edit: actually temp was inside box, not sensor. Exif temp test is
are here.