Get an old VW bug or other car, possibly bigger. Reinforce it with your choice of strong materials, to withstand flying debris mostly. Place it in the ground and cement it there. Voile! an inexpensive way to survive the initial jolt. Call it "Car-boat in cement, make it to withstand liquefaction etc. Seatbelts already there. Of course take out the gasoline tank, and anything else that may explode under heat and pressure.
Offered by Cindy.
Good Idea, This combines the best of two types of survival ideas into one. The curvature of most automobile bodies combined with wire mesh (5-8 layer of chicken wire) reinforced concrete 4-6" thick with some reinforcing rods should make it strong enough. This would be a much easier way of supporting the sides and top than using plywood. Re using the trunk as an escape hatch. This is not a good idea - too easy to get blocked and you may not be able to hinge it open. You need a small strong hatch that opens to the inside. Might be better to make a hole in the roof or trunk, bolt a heavy steel plate (say 1/8" to 1/4") from the inside. Concrete the outside all but this hole which would be slightly above ground level. This hatch might look like a smaller version of a manhole cover from the inside with nuts on studs around the perimeter. The studs would be well anchored, being welded to the reinforcing rods in the concrete.
If the car seats are planned to be used with the existing seat belts then the car could be orientated with the front end of the car higher than the back. This would give less strain to the occupants for forward jolts. This means that the access would be best from the top or the front engine compartment or in the case of a VW bug the front trunk. Note: The VW bugs or beetles I have been familiar with, do not have very strong seats. It would be wise to beef these up well ahead of time if you plan to use them in this fashion.
Offered by Mike.