My 13-year-old son and I built a snow cave this afternoon and are going to sleep in it tonight. I used a 4' x 8' sheet of thin ABS plastic for the interior (like a quonset hut) and heaped snow on top and stacked it on the open sides. We have about 4' square inside with a 3' ceiling at the peak. It'll be tight, but it's going down to 19 degrees tonight, so that's not a bad thing. Just for fun, I ran some wire between the cave and my bedroom so we can talk to my wife on field phones.