Buy oven roasting turkey bags, or oven roasting poultry bags.
Fill the jar with something that only needs heated, like a can of soup. Put on the lid. It won't come to a boil, it will only heat up to about 160 degrees.
Puff the bag around the jar. Seal it with a twist tie. Set the jar on something to insulate it from the ground (mine is setting on a block of wood.) Set it in full sun. (This will not work on a cloudy day.)
If you dump a can of soup into this jar, it will be hot enough to eat in about a half an hour.