Replacing stripped subframe rivnut te450

I’ve searched here and googled it, searched you tube but it looks like no one has ever stripped a rivnut and replaced it in a subframe.
Gotta 09 te450 side panel rear mount rivnut that was stripped. Trying to find a way to remove it to replace it. With the subframe that’s been welded closed so the when countersink drilled to help remove it, the main crimped part will remain inside the subframe.
Has anyone been able to replace the rivnut for the side panel rear mounting bolt? And able to remove the crimped part after using a countersink to remove the outer flange for removal? How did you get the damaged part out of the subframe? Or does it rattle inside the subframe for eternity? Thanks for any help as I’m trying to help my friend fix his 450 after running a rear fender rack.

I took a riv nut tool and just tightened the spinning one in my subframe.

They talk about your subframe issue here…
http://www.cafehusky.com/threads/te310-tank-insert-repair-stripped.21285/

https://www.thumpertalk.com/forums/topic/548923-loose-insert-in-subframe/

Thanks for the link to the fix was almost going to buy the Astro tool to tighten up the Riv nut