I really enjoyed reading this thread. I have been brewing for almost two years and have gone through this kind of obsession more times than I can remember. I have now reached a stage where I trust my setup and know that a small fluctuation is not the end of the world.
We live in a tiny house with two small children, so the only space for FVs is in my shed (no electricity, so a fridge is not an option). I bought 50mm insulation sheets and built a fermentation box using an old computer desk. Take a look at this:
[ame]https://www.youtube.com/watch?v=kS_ETRx7Byg[/ame]
Regardless of the time of the year, I can ferment lagers or ales relatively close to the recommended temperatures. During the summer, I keep two 2L bottles in the freezer. I rotate them between the freezer and my fermentation box every 12 hours. During the height of summer, I add a couple of ice packs from the freezer as well. During the winter, I do the same with two hot water bottles. Both bottles are placed inside the box, with something between the bottle and the glass FV. It works a treat. I have a temperature guage inside the box, which also tells me the temp inside the shed. The temp can easily stay within 2 degrees while the temp in the shed can jump by more then 20 degrees either way. I'm sure the actual temp inside the FV is even more stable.
I have also done the same (ice or hot water bottles) in a large plastic tub filled with water. This does not work as well, but did the trick when I used the box in the summer to keep bottled beer and cider from getting too hot while needing a place for FVs as well.
I did not mean to take over this thread, but I hope that sharing other temperature control options would be useful to someone. We are going to be moving to a larger house later this year where I can keep this insulated box in a garage. I'm sure it would be even more effective then.