I feel your winter weather pain. It's been the coldest winter I ever remember here in MA.
I ended up buying 1/2" foam board insulation and basically cut pieces to size to essentially wrap my enclosures. I covered the back, sides and top and taped the back and sides together so it just slides over the enclosure, and it has helped immensely.
When spring rolls around I'll remove them, label and store them and be good for next winter. Although by Then I hope I've switched over to pvc...