XML : Symfony2 Fosrestbundle custom xml root element

When returning XML from an API endpoint in Symfony2 using FosRestBundle I am trying to change the root XML element to something more flexible (this may not always be an Entity so want to do this without the JMSSerializerBundle.

Currently returning:

  <response>      <capacity>10</capacity>      <availability>10</availability>  </response>    

I would like it to return:

  <mycustomrootelement>      <capacity>10</capacity>      <availability>10</availability>  </mycustomrootelement>    

I am not sure if I can register a custom service to do this, maybe extend the default XMLEncoder or similar and then configure this in the FOS configuration.

At the moment I am just returning the response and can not see anyway to customize this.

Has anyone achieved this - some advice would be GRAND!!