I really like the idea of carrying a spare parts, although some experiences lead me to believe that the one you do not have is the one that will be needed. When I was about 5, my family spent some time on the side of the road when a hose blew. the one hose not replaced prior to the 1,400 mile trip, from CT to FL.

However, if you are bringing the parts, don't forget the ancillaries. What good is a new gasket, if you cannot seal the fitting that covers the thermostat. In that vein, some high-temperature silicon might be a good addition to a car kit. Instead of having to carry gaskets, you can use it. Some silicon sealants are good at up to 500 degrees, so it could work for a while and let you limp in, or go further. Something like:
http://3m.hillas.com/pc-15450-2163-3m-08672-ultrapro-high-temp-silicone-gasket-black-3-oz-tube.aspx