It would depend on how many people & pets are in the shelter. Also, how tight the entrance door is. If there is an open flame, that will make a difference also.
You could put a 6" PVC pipe through the ceiling and put the bottom close to the floor for the intake. For the exhaust, use 8, 10, or 12" PVC in the opposite corner of shelter with the pipe close to the ceiling. Paint the exhaust pipe black and wrap it with clear corrgated fiberglass. During the day, the heated large exhaust pipe will draw air out and let the fresh air in. During the night, use a 12V fan from a battery that was on a solar charger.
Do a search on bunkers. There is a lot of information on the forum about them.
The attic spinner fans could be used also.