Sue,
Now I like this one better than the hot rocks idea <img src="/images/graemlins/smile.gif" alt="" /> . AND, speaking of the hot rocks, during your thread there, someone (maybe you?) did mention setting up a tent indoors to contain body heat as opposed to trying to heat an entire room. So, I personally give this one a thumbs up.

As far as the sides falling over because they were bumped into, well, nothing says a person can;t use whatever reinforcements they have at hand to help sturdy the shelter. I take from Sue's post that matresses ( and other things ) can be used to minimize heat loss, however you choose to contruct them or put them together.