Hi JDS, I used a similar method on my noid fig:it was only 2' × 2', so wrapped it with opaque plastic (old peat moss bags, split), then poured in a 4" layer dry peat moss mixed with red chili pepper to keep vermin out. I filled the area to the top with more dry peat moss, and laid a sheet of plastic over the whole thing, weighed down around the edges with stones and soil.
After substantial snow, I piled a good layer of snow on top for added insulation. Come March, after an unusually mild winter no lower than 10°F, there was almost no damage at all. Hopefully this setup will survive the acid test of -10°F, which is more typical.