I’ve been looking into Sodium Acetate as a long-term heat store. It’s a phase change reaction, so this heater could be warmed on your woodstove and then left in situ for weeks or months, so long as you remember to check the level now and again.
This is a concept, I haven’t built it yet
A couple of large pots stacked on top of each other,
10kg Sodium Bicarbonate,
When the temp drops below 1c the pump activates and dumps some Sodium Acetate from the top pan to the bottom one releasing heat. When the temperature rises the pump stops.
The thermostat could also activate a 12v fan to move the heat over a larger area.
If you heat the thing on a biochar stove it could even be Carbon Positive
Or you could use a couple of hundred watts of solar to heat a 12v kettle (100w or so)