David has offered some expert advice on doing fender flares, and if you look through his work on this forum you will recognize his abilities in magically transforming simple styrene into works of art. Another possible idea is to use white plastic spoons as the flares rather than the flat stock. It will still require some filling and sanding, but it gives the "bubble" effect just a bit better. Not trying to take away from David's idea, just offering another alternative.