Couldnt agree more ref wiggys bags. Ive got two of em. One is the -30 Ultima Thule with a hood, rectangular bag, which I used inside a jerven kingsize bag in the arctic, and I think its fair to say the other people on the course were quite envious. Heeheheheee. It had MASSIVE loft. I took up so much room in the tipi.
Just that under a tree, is all you need. No tent or tarp.
And the other is the square rectangular ultra light hunter. Again, just soooo comfortable. I use that in a double sized Oware bivy bag, made from quartek upper and sil bottom, with mossy mesh.
I dont sleep well, but Im out like a light in five minutes in those. They're brilliant bags.