One 40 Watt light bulb in a car mechanics inspection lamp slid under the cowling for a couple of hours before the flight.
This leads one to think of a more effective way.
Th crankcase and sump and made of aluminium, and I wonder if there is a product based on a heating element which could be attached to one of the (many) screws which are all over the place. Such a heater would be left permanently attached, and would be wired to a connector somewhere. A small device could easily dump 100-200 watts into the engine in this way.
Of course it depends on how much one is trying to heat the engine up by. The UK is rarely that cold... -2C right now (Sussex).
The installation would have to be signed off - a Minor Alteration would probably do.