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Wow, erhm. If you want to slowly get 2nd degree burns then this is a good idea. Waking up with a 1ft blister on your back is not as fun as it seems.
OR you could just heat up fist-sized rocks and put them around you before you sleep. IIRC they should be warm for 2-3 hours.
For you defending his design: look at it again. It is a 2ft deep hole. With a fire at the bottom. (inb4 muh embers)