As Dean himself pointed out years later, these days, if you are preparing ahead, you can do even better. HEPA air filters are common, and with them you can make a better air pump. More efficienct and easy to change the filters.

Standard fittings for furnace filters will work fine. I'd have the air entry chimney have a long vertical shaft with the air intake above the bottom and a trap to let you shovel out accumulated dust from the bottom (said trap to be *outside* the inner shelter, thank you very much).