Very interesting device. Thanks for sharing that. Reminds me of the setup George Sears (Nessmuk) described for his Adirondack travels. He would use a canvas Forrester tent or a debris shelter. Both are the same general shape as your tarp. In front he would have a fire that refected into the shelter. To increase heat reflection into the tent, he would build a log "wall" behind the fire. I bet you could also do something similar to increase heat relection. He wrote about these methods in 1880s, and they are still in print. Good ideas never change!