Okay, here I go and I'll try to be coherent:

Air Ventilation: The air filter that I'm using is a Caterpillar make, used on extra large earth moving (dump) trucks (V-16 diesel engines) with a coarse outer filter element fitted around it, and I fit a NEW shirt (flannel) over that. I mount the filter onto a plywood base and connect 4" pvc piping to it for the ducting running inside the garage. I've got a 110vac squirrel cage fan that sucks air into the shelter via the filter.
I also fit a large piece of plywood over the top of the filter to give it some protection from the rain.

Power: The 110vac comes through a inverter, hooked to a 12vdc battery. I have a magneto that is driven by an appropriately mounted bicycle's rear tire to recharge the batteries. I am also looking at getting some solar panels and/or small wind generator to do the same thing.

CO2 removal: I run 2" pvc piping from the flooring area, from 1/4" above the floor, to the outside and then upwards a few feet. The top of the 2" pvc piping has a simple rubber flapper valve afixed to it. The simple flapper valve consists of a piece of rubber cut in a 3" diameter and stapled into the top outer edge of the pvc piping.

Operation: Energize the squirrel cage fan to suck air INTO the room via the outside filter. Incoming air creates positive pressure that finds release via the CO2 pipe. CO2 (& Carbon Monoxide) are heavier then air. The incoming air forces the low lying CO2 into the exhaust outlet. The forced outflow opens the flapper valve. When the air intake fan is shutoff, the outflow stops, and the flapper valve closes.

All of these parts are assembled in sections and/or stowed in my garage within easy reach so that they can be assembled and sealed together quickly. That also includes the plywood sheets (precut) that I use to seal the garage doors and windows.

I also have to seal around the entrances to the attic and into the house to make this work properly. Another item that has to be accomplished: sealing/bagging all of the roof vents. Sealing up of the house is imperative to prevent contamination of the home while sheltering in the garage.