I feel like I've seen auxillary heaters for sale somewhere. Also I've seen heaters for hot rods that are basically just a box to set under the seat. Thinking about this it occurs to me that I could simply use an extra truck heater...

1955 1st GMC Suburban | 1954 GMC 250 trailer puller project | 1954 GMC 250 Hydra-Matic | 1954 Chevy 3100 . 1947 Chevy COE | and more...
It's true. I really don't do anything but browse the Internet looking for trouble...