I spotted this modification in a magazine article about Destrys KX500 but since I bought his secondhand rear subframe recently I can see the evidence with my own eyes.
It didn't come with the angled bit of aluminium but the holes were already drilled for it.
These pictures should take you through most of making one for yourself.
I will post the fitted picture after this weekend when I have got some bolts with the right thread for the existing holes in the subframe.