I can't find the website that offered it, but this is a fantastic idea!
How about a Styrofoam cake? You can easily decorate it to your own tastes.
The twist though, is you put in one little space where you can insert a real slice of cake that you and your partner can cut together.
Meanwhile, in the back unbeknownst to everyone else, you have a sheetcake ready to be cut.
Super cheap, super stealthy, super easy!