After several iterations, I've come up with a very functional design. Let's look at some data and see what it took to get it right...

... third time's a charm!

• Trial 1 was a 300W aquarium heater, in a 5 gallon bucket of water, sans insulation.
  
  • As you can see, the rate of heat gain wasn't that great, and thermal losses mounted, slowing gain down even more.
  • I gave up well before reaching the target temp (140 F).
• Trial 2 was the same as Trial 1, with the addition of copious amounts of insulation (foam-in-a-can), side and bottom.
  
  • Again, heat gain wasn't all that fast, but the added insulation made a big difference in the heat loss.
• Trial 3, now that's the ticket! I purchased 2 more 300W aquarium heaters ($8.00/per), for a total of 900 watts of heating.
  
  • The 5 gallon bucket heated to the target temperature (140 F) in roughly an hour.
  • After reaching target, my software shut down the heaters in order to measure the heat loss rate. Not too bad.
  • The PSSR/ZC tail isn't rated for 900W, only 450W, so I switched it out for a standard PowerSwitch Tail, which is rated to 15 amps.