Hi figgi11,
You need at least a sort of greenhouse to keep the heat down around the fruits or you will just try with no success to help global warming.
I would use plastic sheeting making a tent over the trees for that. Of course if your trees are gigantic ... it is perhaps time to plan to keep them smaller for the future.
I wouldn't use a space heater outside without a roof to keep dew and rain out of the heater electrical parts.
At some point, you should consider the last figs to be lost especially this time of the year and in a cold zone.
Just watch them ripen or not, with luck they might do it without the heating system and the risks involved.
Or perhaps time to build a greenhouse around your trees :)