I built this thing just over a week ago, and it works like a champ.
http://brewstands.com/fermentation-heater.html
Just brewed my first Tripel a few weeks ago, and then a cold snap brought my ferment down to the mid 50's. I ended up seemingly stuck at 1.035 and ended up having to pitch some High Gravity Yeast yesterday. The new heater has been keeping it right at 70 for the last week, however.


