Stools are too slow for air filling. I will use one for a brake job.
For air I kneel using a kneepad, I have 2, if I can't find either I use a folded piece of corrugated cardboard (twice as thick) to kneel on. Since the recycling bin is in the garage, and we're always buying stuff, there's always cardboard. Also I keep a few large sheets behind the rolling toolbox to lay on under the vehicle, much more comfy than concrete. I cut them with a boxcutter to the size of the back of the toolbox, once stored they take up pretty much zero space.