I think the key in CO tests is keeping the top up. That canvas is seriously
porous, even without  holes like mine has, and it provides a huge amount of
air transfer that you wouldn't get in a regular car with the windows up.

It also strikes me that two adults, a child, and a dog will convert a large
amount of O2 to CO just by breathing, maybe more than a small propane
flame--I don't know.