If you get some blue foam, shape it, cover it in glass cloth and resin and then, when its cured (next day) you can dissolve the foam away completey with thinners leaving you with a nice thin GRP shell and a very messy floor. You still have to fill and sand of course but it works well. Takes a lot of thinners though.
I wouldn't recommend using petrol to dissolve polystyrene as that is how you make napalm interesting body making technique though. I am still deciding what to make my bodywork out of so its interesting to see/hear a few more options
I do know some people, when they use large cell polystyrene, wrap it in brown packaging tape to protect it from the resin. Please note I havnt tried this myself.