eberspachers are very good but a little expensive, Mike made some excellent suggestions some of which we use, ie Old Quilt over machine, however we just use a Parafin greenhouse heater on the lowest setting, it cannot fall over becuase of the shape of the base and the flames arent naked as they are covered by metal tubes, we just make sure nothing is near by and leave it on lowest setting to just keep it above freezing.
Dene