I didn't read all 53 replies, so I hope I'm not repeating anything.

For the first poster, don't forget air holes so you don't suffocate.
Then again, that would defeat the waterproof aspect for which you ask.

For all posters, don't forget to tell others ahead of time what you
have so they know to look for you when you can't get out after the
storm because of the car that was blown onto your shelter door.