Watch how to take apart your xbox on youtube, there's hundreds of videos of how to do it.
1. Buy a dremel tool, or hand rotary tool.
2. once you have your xbox open, pencil out your design on the case.
3. whilst wearing safety goggles, use the hand rotary tool to cut out your design.
4. cut a sheet of perspex to required size and use hot glue to keep in place.
5. reassemble and enjoy!