Polystyrene is light so you can use it to make large parts that otherwise (if you used sugar paste) would be very heavy. Polystyrene is not edible. If you want the cake topper to be food safe, stick to the guidelines below.
1. Cut and smooth the polystyrene away from all edibles.
2. The polystyrene needs to be isolated from the cake (with sugarpaste or foam board).