In order to hide the default radio button, I'd suggest to remove the button instead of making it transparent as all visual feedback is handled by the drawable background :
android:button="@null"
Also it would be better to use styles as there are several radio buttons :
<RadioButton style="@style/RadioButtonStyle" ... />
<style name="RadioButtonStyle" parent="@android:style/Widget.CompoundButton">
<item name="android:background">@drawable/customButtonBackground</item>
<item name="android:button">@null</item>
</style>
You'll need the Seslyn customButtonBackground drawable too.