You know those electronic thermometers that read both indoor and outdoor temp and have the remote wireless sensors. Has anyone put the sensor inside with your fig that was all wrapped up? That way you could see how cold your fig is inside the cocoon you built around it. If you use one that has the min and max button, you could see how cold, or warm, it got inside during the day.

here is a link to one that is supposed to transmit 100 feet as an example

http://www.acurite.com/00782-wireless-thermometer.html      

I did a summer monitoring:
http://figs4funforum.websitetoolbox.com/post/measuring-the-soil-temp-in-the-pots-6480114?highlight=temperature+pots&pid=1279344148#post1279344148

Very easy and you can record the temps for the entire winter and plot against the outside temp if you want.

I have been using it in Zone 5a to monitor inside the garage where figs are stored for the winter . I want  to make sure it is just close to freezing (32F/0C) inside the garage during the winter when most of the mid-winter it is around -20C outside the garage. It saves me $$ by adjusting the thermostat to keep the desired temperature.