It's not that hard to find acouple of plans off the net, same goes for filtration systems that can be pumped manualy if eletrical fails. one of the main thing with filtration of outside air are the presure shut off vales.
I'm not to familiar about it but after the initial blast it deals with the presure issue off the blow by.
En-mass your patriot neighbors, friends, family, prepare tactical, and storm troop one of the many existing D.U.M.B's around your area. ;)
You may already be underground by now from the age of this topic ;)