Wow, I love dunkel and I've wanted to try making one for over a year now so this thread is great! Its too cold here to ferment in the brew shed yet and I don't have enough room in the house or a place I can keep the temperature around 45* (without my wife complaining ) so I'll have to wait a month or so probably to make a lager. Which yeast is best yeast for it? I like Warsteiner it has a very smooth mouthfeel to it.
Try an aquarium heater.
I use one in a swamp "cooler" in my basement with Winters icy grip upon us.
Was using an old one from a fish tank we had but it crapped out. Wal-Mart had a nice short one that works perfect.
Make sure to get one large enough. They size them according to how many gallons they can heat. Mine is a 15-30 gallon heater, works great.
You'll want a temperature controller because almost all the new aquarium heaters I saw have a built in thermostat to keep the water at 76-78 degrees.
Its also nice to have one for Belgian styles so you can ramp temps up as fermentation moves along.