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!
It works until a float sinks, the carb floods, and avgas drips on the light or fills the cowl with vapor. It works until the light finally blows, or a short occurs, or something else happens to cause a fire. It happens
Maybe they don't have these in the US, but they are 60W electric oil filled and you can pick them up with your hand as the surface temp is way less than 100 degC.
They could probably be dowsed in petrol without any problem.
They do still cause condensation in the top of the engine.