There you go
Opening a window low in the house at the same time you open the attic access should let the chimney keep functioning properly.
I have a window next to the stove cracked a bit and I'm in process of installing a fresh air intake for my oil burner so I have the intake hood installed but the pipe just hanging there.
There is a fair amount of air coming into the basement.
The family was heading out and I had a feeling I knew what was going to happen. Sure enough, the CO detector was going off when we got home. Had to open up all the doors to air things out.
I had to reinstall that attic panel and everything is working normally now.