You might making a sacrificial element that gets cast and that extends past the medallion. The metal will flow past your design and slide the contraction of metal from the middle. It’s probably at the limit of what your centrifuge will do already. Think of adding something that will take the extra metal you need and pull it back INTO your piece as it cools. Maybe a smaller main sprue or doing to two. An extra crank of the spring. I always do 3. Sometimes you have to fool the casting to adjust the metal pulling from the thickest part (or somewhere inconvenient). It’s more cleanup but it might help.
Large, thin …and flat shapes are somewhat challenging.
Good luck.