With a serrated knife and using a sawing motion, cut a sponge into two cake wedge shapes to make the postcard. Large sponges can be found at hardware stores and in the automotive departments of most big discount stores.
When the sponge wedges are dry, trace around them on a piece of cardboard, craft paper or colorful scrapbook paper. This will be the postcard side. Heavier paper works better and will give you a sturdier wedge of cake for mailing. Cut the postcard shapes from the paper.
Using a caulking gun and your desired caulk, fill a plastic disposable piping bag with half of the caulk. This should be enough to do one cake wedge. I was able to pipe two cake postcards with one tube of caulk, but I recommend putting only half of the caulk in the bag at a time. Also, I tried decorating my cake wedges using the caulking gun but it was nearly impossible to control.
Pipe dollops and swirls of caulking on the top and sides of the sponge wedge just like you're decorating a cake. Pipe along the edge of the cake postcard to form a seal between it and the sponge. If needed, use a small paintbrush or toothpick to swirl and adjust the thickness of the caulk on the cake top and side.