look up things like "vintage Red Head" and other brands of stand alone heaters. "6 volt Heater" and "auxiliary heater" are some other good eBay searches. That should lead you in some good directions. There companies that make them for vertical or horizontal mounting of various sizes and of course, prices.
Here's a link to a modern company And another You could wire another cigarette outlet plug in the rear and run a little cheapy that wouldn't have to be mounted permanently. All sorts of ways to skin this cat.
