Hi Ray,
I'm late to the party but I did build and use earthtainers the first time this year. I started my first 2010 tomatoes in February with a 100w aquarium heater in the reservoir. I got good growth, but no early tomatoes.
For your incutainer, my first suggestion is to insulate. How about taking two more of the waste container bodies, split, and use them as outer shells. Put reflextex or sheet styrofoam between the shell and container and top. I would just bungee the shells together, but a more elegant system may be desired. Just to note that the overflow ports should be extended through the insulation and shell. Also set the incutainer on a sheet of insulating foam.
Once the container is insulated, your heating requirement should be significantly reduced. You have the aquarium heater for the water. Much of this heat will now flow upward through the media and keep it warm.
Your concern is nightime temperatures. Kinda like we are having now (28 deg in Sacramento). I would first try one or two 23 watt CFL's on timers inside the top. This would give the added benefit of extending the light growing hours. Another idea is to mount a seed heat mat vertical inside one side of the top.
I currently add a bit of heat to my coldframes with lightbulbs on cold nights. They also have vent openers on the covers to ventilate when the sun is out. A vent opener on your cover may help put your system in hands free mode.
Enough for now, hope this helps,
And thanks for the great job designing and instructing everyone on building earthtainers.
Rick
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