Rather than buy a special bowl for baking half-circles, you can use a Pyrex bowl to create the same effect. The end result is cheaper and pretty much the same as long as you follow a few guidelines for baking in a Pyrex bowl.
Make sure your bowl is oven- or heat-safe. It should say so on the bottom.
Grease and flour the bowl generously. Shake out any excess flour. This will ensure that the cake doesn't stick. Because this cake will be heavy and large, this will make sure you don't lose pieces.
Mix your cake ingredients, and pour them into the bowl. Put the Pyrex with the cake mix inside in the oven.
Preheat your oven to -3.89 degrees C below the temperature suggested on the recipe.
Bake the cake as usual, using a wooden skewer to check its progress. Because of its size and shape, you will have to cook the cake for longer to make sure that the entire thing cooks evenly.
Make a test cake if you can. This will ensure that the cake comes out successfully for the birthday or event you're celebrating.