PL, It's a good idea to protect your tree. Hardy is a relative term. Your tree roots might survive unprotected, but the top will die back. You don't say what variety your fig is. Some figs will grow back from the roots and produce a crop, others won't. The problem in Zone 5 is that even if your fig is able to produce after dieback, it will probably start so late in the spring that the fruit won't have time to ripen. A late start is one of the disadvantages of inground figs in cold climates. It's a good idea to pick a site that warms up quickly in the spring and help it along with heat trapping techniques.
There are many ways to protect figs (and strong opinions to go with all of them, LOL). In my opinion the goal is not to keep the tree warm - nature has provided the protection of dormancy which is triggered by keeping the tree cold. You are keeping the temperature from fluctuating wildly. One of the toughest parts of the protection problem is in the spring when the sun starts getting warm and your shelter heats up. Heat at this point can cause as many problems as cold. Any moisture inside will encourage mildew.
My preference for winter protection is a vapor barrier over the ground around the tree, insulation, and a reflective covering. The vapor barrier and insulation can be achieved by filling large garbage bags with dry leaves and stacking them around the tree once it has been wrapped with fabric (rug, burlap, etc.). Don't let any plastic touch the tree. I usually don't cut back, but many do (to make the wrapping easier and to get cuttings that otherwise might winter-kill). The whole pile can be covered by reflective material(scottsmith used aluminum bubble wrap - a search will bring up his results)or a reflective tarp which will keep the temperature inside warmer in the winter and cooler in the spring.
The bucket method was to cover a hole that was left in the top of a waterproof tarp covering the tree and its insulation. The hole was to let the structure breathe when it heated up in the spring. The bucket was to keep rain from going in the hole.
As I mentioned the above method is only one way to go about winter protection - a search will provide others.
Good luck and many figs next year!