I have a 60W greenhouse tubular heater resting inside the bottom cowling of my Moth (and switched on), which keeps my Gipsy engine warm enough to start no problem all Winter. Works for me!
There is a downside to this as it will heat the oil in the bottom of the engine. This will drive off any water or acid in the oil which will then condense on the top of the engine which is cold.
This then means that the top of the engine can corrode. Unfortunately this happens to be the camshaft on a Lycoming. Not sure on a Gypsy, but I think it might be the crankshaft.
Lycoming specifically advise against sump heaters being used continuously and only recommend using them just before starting.
I have heard about people who have sump heaters controlled from their mobile phones. This allows them to be switched on a few hrs before you get to the hangar. Does anyone know of any switches that can be controlled by mobile phone as this would allow a very simple remote controlled fan heater to be rigged up.