I have a very simple system that I use.
I have a ceramic heater on the floor under the cowl that has an aluminum "plenum" of sorts riveted to it. From that, I run a length of aluminum dryer vent, the flexible kind, up to the cowl and insert it between the exhaust.
I plug this into a thermostat that is intended for heat tape to keep pipes from freezing. The thermostat is placed into the cowl through the oil door. Then plug the thermostat into an extention chord.
Then I place several blankets over the cowl and walk away. I keep this rig plugged in all winter and it keeps the entire engine compartment warm to around 40 F. When temps are in the teens, it runs maybe 3 times/hr and usually about 1-2 minutes each time. Engine starts up very easy. The thing that I will start doing, however, is doing about a half hour preheat with another ceramic heater inside the cockpit, as my electonics tend to say "yeah, right!" when it is cold.
REgards,