Golden Rod.
Very low watt density (which means its not very hot to the touch) but still hot enough to keep the safe a little warmer than ambient.
Try to put it down as low in the safe as you can. Promotes natural circulation. If you put it up high, all the warm air will stay up in the top of the safe.
Or, a very low wattage incandescent light (25 watts or so) will also work, but as I'm sure you know, the surface temperature of a light bulb is pretty hot, so don't let anything near it when its on.