Awesome reminder and great list! I've been meaning to add stuff like this to the back of my X. To help me finally get around to it, I've just ordered one of these from Amazon:
http://smile.amazon.com/Backseat-Org...A1KHAUAXGV6GPP
In addition to your list, I'm planning to keep a couple of 3 day rations and some Mainstay or Datrex water packets. The water packets are nice because they are more stable in heat and cold than other water storage options. I'm also going to keep a change of clothes (some old jeans, t-shirt, socks, and underwear).