I picked up a 4x8 sheet of polystyrene for $19, yay plastic supply store...

Anyhoo I did some rear deck work, rear end and some minor bits with the styrene. I also picked up a tube of finishing filler to fill spaces and gaps that I don't want. I've never used a filler / putty before so I'm learning as I go. The stuff says it's sandable and paintable so with luck it'll be fine. It goes on pink and dries / hardens white!