After spending around five hours in the R2 Builders chat last night, I got back to work on the outer foot shells.
I sanded down the Bondo that I applied Thursday night, again by using sand paper wrapped around the battery box, and running the curved part of the foot shell back and forth.
With so many different colors to look at (gray and white for the PVC, pink for the Bondo), I used some gray primer to help find the remaining flaws.
Things actually look better than I thought they would, but I still have a little more filling and sanding to do. I'm ignoring the middle area where the curve and flat part above the curve meet, since that is going to get cut out for the drive train to fit into the battery boxes.