Try to do something like this:
<?xml version="1.0" encoding="utf-8"?>
<layer-list xmlns:android="http://schemas.android.com/apk/res/android">
<item android:bottom="-20dp" android:left="-20dp">
<shape android:shape="rectangle">
<solid android:color="@color/white" />
<corners android:radius="20dp" />
</shape>
</item>
</layer-list>
It seems does not suitable to set different corner radius of rectangle. So you can use this hack.