Here are some simple icon designs. I like how easy clean they are and each one has a specific opening to it that allows the icon to be a part of the background. I also like how they are all see through and tie in well the background also.
Secret to flat cakes: wet cake strips (i., Bake Even Strips) and wrap around the cake pans. They act as an insulator to help the cake bake evenly (flat). You can also use strips of towel: drench, wrap around the pans, and bake.