I would like a Webasto diesel heater but they are very expensive for the universal ones which can be fitted to any car. There are some on ebay from rover 75s but from what I have read they are designed to connect direct to the cars wiring loom and cant easily be altered. There are also some on ebay off Disco 3s but I suspect they have similar issues to the rover 75 ones unless you have a disco 3 that is.
I did think about sticking an immersion heater in a piece of tube with a pipe in one end and out the other and a central heating pump and plumbing it in to my engine bay and plugging it in to a time switch.
I make biodiesel and have done virtually the same thing to heat my processor.