Weld ground down. You have to resist the urge to grind it down nice and flat as you are thinning the metal out by doing that


This is under the top skin I cut away over the rear mounts
I drilled up through the mounts and the upper skin

You can see the hole highlighted here


In the top of the pic you can see the rear chassis leg.

It's hard to believe that bmw could be so stupid as to not tie these together
Instead the loads are transferred out to spot welds on the other panels. Also because it is so far from any fixed point it is going to flex a lot more.
It's very clear that welding on plates as per reddish etc will not help this situation. It will only kick it down the road till the whole panel pulls out of the car.

Here is a video of a boot floor on a car with 40k miles and bmw structural foam injected. It's amazing that they last as long as the do when you see this

https://www.youtube.com/watch?v=ee0VvI8SlEQ

I made up some plates to transfer the load to the chassis leg




Here is the top of the rear mount highlighted again. It a lot nearer to a structural point now.
This is not enough on its own as over time the flexing will fatigue around the bottom welds. It could be spread out by plating around here to spread out the load.
I'm going to use longer bolts up through the rear mount and then tie that into this area and the cage on this particular car