Sewer pipe and or precast concrete. Those large concrete sections are small arms resistant able to run through. AND if placed correctly can be used as storage or extra bunkers. With pre planning you can set up your own shelter by pouring it your self. There are places that also deal in recycled concrete, this would give you a moderately fortified bunker area.
Give some consideration to the stainless 3' diameter lines out there. Not a run through in hurry, but you would still have emergency outlets as needed.
Here are a few links for both fiberglass and concrete.
http://www.stormsaferoom.com/belowground/index.htm
http://protectionshelters.com/storm-...s/above-ground
http://vaughnconcreteproducts.com/storm-shelters1