Let me just make one point in case it isn't obvious. The quantity of water in your "bain" is key. If you use a small heater or a small amount of water, the temperature will drift more quickly or be more susceptible to variation when you put a container in. So, to be safe, you want the "bain" water to be many times more voluminous than your developer and container. Then your temp will stabilize and stay constant for a long time. This is why I use the food service things; I could put a tray of icecubes in there and it wouldn't change the temp. It's overkill for some purposes, but I do E6 up to 8x10 (and soon 11x14) so... I had to worry about that.