Using a hot styrofoam knife I use a 10 x 3 round to cut cake slices from. I then use Spackling to frost the sides. I use a scrap piece of styrofoam to make the icing look textured, you could also use a fork for this. Let the spackle dry at least a day or two and then paint and pipe on details.
Fake cake slices are available for sale in my Etsy shop