Hmm, just to go back to the original scenario: I'm not sure it would be wise to use anything but a couple of candles to warm the interior of a vehicle. Too much flammable stuff in there, and not enough air. You would need to crack a window for ventilation anyway; the more smoke generated, the more ventilation needed, so you'd lose the extra heat. Stoves belong outside. I'd put a tarp off one side of the car, bottom enclosed with snow, for a stove (and/or bathroom) nook out of the wind. That's my take.

But back to the question:

If you were serious about the plywood, you could staple/tack a layer or two of aluminum on top of it, shiny side out. This will reflect the IR and conduct heat away from the wood. Free sources: pop cans, scraps of flashing from construction.