you can get hot/cold boxes that plug into the cigarette lighter if you are going to do it a lot, otherwise maybe get a cheapo microwave and heat it up when you get there?
The insulated picnic boxes used to keep food cold will also keep food warm. Warm it up first with a hot water bottle or similar and it should keep your hot meal nice and warm.
Cheapest option - wrap plated hot meal in several layers of newspaper and place in a smallish "coolbox" or "coolbag" - which will, of course, keep things hot, too. Drive like the clappers.