Routers are amazing tools, but they are not forgiving. Make a design template and use that to create accurate and identical pieces, each time.
Use a 6mm piece of hardboard as your template. Trace your shape onto it and cut it using a jigsaw or small circular saw with very fine teeth. Sand over the edges of your template to fine tune it and then attach it to your piece of wood using double sided tape.
You'll need a flush trim bit on your router to shape your piece and a bandsaw can be helpful to make the first cut. See the full demonstration in the video above.
Pattern or Template Making with a Router [WoodWorkWeb (YouTube)]