This article is from the Baby Proofing FAQ, by Sandra Smith, firstname.lastname@example.org with numerous contributions by others.
From: email@example.com (Chris Phillips)
We never use our fireplace. So I covered the whole lower part with
cardboard (leftover from moving boxes). It is still somewhat hard, but
at least is not as bad as brick or stone. It also helps a little to have
the bends in the cardboard a bit away from the corners they cover. You
would have to take it away when you had a fire (it could catch fire).