Needed: Ideas for a cheap temporary Floor work surface

One question, are you talking for a weekend while you work on your car, or like to sit on jack stands all winter while you work on it? End solution for the car sitting on jack stands for months may be different than sitting on them for a few days.

leave the dirt floor, you can set the jack stands on a piece of steel plate that is setting on the plywood (or just steel plate as suggested) and get a piece of scrap carpet someone pulled out of a room or something, Cut the carpet into 30-36" strips and roll it out to lay on... roll it back up when done... CHEAP....

If you want a bit better, get a couple yards of 5/8 crush gravel and spread that around level and tamp it down or roll it. That will give a good base for the steel plates on the bottom of your jack stands....

I have even set jack stands on old particle board shelving with melamite on it so they did not sink into my blacktop. The particle board was fine...