Daintree, i didnt fix the styrofoam house to the ground in any way , it was so tight on the canopy and the friction from the branches kept it quite sturdy, i just placed some soil at the bottom to fill up the holes i had.
I dont know to what an extend the box helped, meaning that the fig right next to the enclosed one managed just fine trought thim mild winter. However, last winter was kinda like the one this year, and then i had almost every tip bud on the covered/back then uncovered fig, die due to frost. This year with the coverage 100% of the tips are healthy and green, wating for the plant to start to wake up. The other one that didnt got protection is awready waking up, i suspect since i removed the coverage on this one a week or so ago, it will wake up later. This may be an advantage, the white box apparently kept it cooler now in spring, thus altering an early wake up. I think i could have left the box on till like the end of april, and the plant would still be dormant by the time i remove it, works kinda like an refrigeration box .
Bussone, i past years i was also doing the winter protection on the inground figs this way, some tightly packed hay, covered with an nylon bag, covered with a white reflecting weaved sack. Worked well, ive also done the ground burying method, also works fine, as long as you have young flexible stemms that you could guide down.
Nice fig plant by the way, mine are smaller than this, will post pick of them tomorrow.