OK, after a somewhat long delay due to the weather, I was finally able to make a little more progress. I enlisted the help of my daughter who has really taken an interest in arcade machines (I think she just likes hanging out with her dad) and she inspected the side that I first filled and sanded. She noticed several imperfections by running her fingers over the surface which she circled with a pencil (thanks Bella!). Then I applied some additional bondo in those spots and sanded everything smooth.
I was pretty satisfied with the results so I did the exact same thing on the other side panel. As you can see in some of the pictures there were several spots along the back edge that needed to be filled in so I clamped a piece of scrap wood behind the panel in order to apply the bondo. Before applying the bondo I wrapped the scrap wood (which was actually fireplace kindling) in plastic so the bondo wouldn't stick to it.
It seems to have worked - I'm not sure how strong it will be but I don't envision that area taking a beating or anything so I think I'll be OK. The tricky part is getting a nice clean edge along the back - I think that would be noticeable if it wasn't exactly straight.
This part sure is taking forever. I still have to go over the entire cab one more time at least (probably 2) filling in the small imperfections with bondo and sanding it smooth. Once that is finished I'm going to go over everything with 220 grit sandpaper to get it ready for the primer. What a pain!