I think it was Gigantohawk that first noticed that the rollers were a little low. He referenced a posting on RC-Panzerketten where they raised the first one 3mm and the rear ones 4mm. He posted pics and talked about it on his build.

You've done already what I was going to suggest, using the styrene to fill the switch hole and putty (I use Great Planes) to fill the smaller ones. I also have some wood filler that I use occaisionally too. Looks good!