I just used a foil-faced bubble wrap, commonly known as "radiant barrier". There was a wooden partition behind the stove, and a unheated root cellar to the left. Even though there was like 6' of clearance around the stove, the walls got HOT when I was really cooking the Hitzer. If I went in the root cellar the concrete wall between me and the stove felt warm to the touch. After I put up the radiant barrier the walls felt cool to the touch, and if you stood in front of the stove...the heat would make your eyes water. I don't know if it saved me any coal, but it did cool down the things I didn't want heated.
As for one above the other, I think it would make sense to put the appliance the requires the most draft on top. In my case, the oil boiler called for -0.05" of WC, and that also worked very well for the Hitzer.