What do you mean?:
You dig it Just once.
Frame the top,and cover,with roofing asphalt paper,and anything else in top.
You will have it for a lifetime.
And that is your only practical option if you want to get ripe figs off your own fig tree.
If you go for the top 2 situation,you might as well find a Farm in California and buy their figs!!!!!
No offense.

newbie, Herman is right, but there might be another option - insulate the figs in a corner of your garage. If this is feasible, I'd take early morning thermometer readings to see just how much difference there is between the inside and outside temps. You are trying to keep temperature from fluctuating quickly - not trying to keep them warm. I've had protected figs go down to 10 degrees F without problem - you don't need to maintain a 30 degree range.
A structure isn't necessary - silver bubble wrap does a great job and can be reused. If the temperature is indeed very cold in the garage, adding covered water filled trash cans (or any mass that resists temperature change)would also help.

Herman, no offense taken. Keeping an open cellar is not my first choice, but maybe, if no other options work out. I guess in reading about fig cellars and burying, I assumed both were done each fall, and you filled in your fig cellar each spring--more of a glorified burying method. But if you put a cover/shed over the top, I see how convenient it would be.

fignut, that is an interesting idea. I'm thinking a wall shared with the house is a good place, with the foil faced bubble wrap ...now the wheels are turning...there is a spot in the corner where the furnace flue goes past, I could capture some of that heat as well. It would be easier to keep it around 25-30F that way.

Thanks guys for the input...
Mark