The extensions that this bracket were created for are found on Amazon here....
I included images of how you could add supports. I flipped it and added support underneath where the arches would be. Although I have had zero movement with mine, you could add a small amount of hot glue on the interior edges of the extensions when you insert them prior to drilling.  

Printed on my MK4, they fit like a glove. Enjoy easier access to your headphone swapping. :)