Having water proof fabric in an emergency situation is not only common sense but could also save your life. This is so simple to do so roll up your sleeves and get busy.
The secret is waterproof and fireproof “tin cloth,” invented in some year by some guy — probably an American — who needed something tougher than canvas or denim — tough enough for fighting dragosaurs with a claw hammer on horseback across the Great Plains. Tin cloth is that kind of tough. And you can master homemade waterproofing by making your own tin cloth.