I do use 12 inch wide boards when I make raised beds. This one sits right on top of what was grass. I too, have wondered and worried about what the soil temperature is like in winter. Before heavy snow (but long after the ground freezes), I place filled mulch bags all around the sides of the box, set as you see here.
The bags extend above the box and help to hold the leaf mulch in place that cover the surface. (Our strong prairie winds tend to blow away leaves that are elevated like that.)
