You could try dremelling off a small amount off the back of the wing mount and fitting a 3mm nut. Doesn't take a lot of plastic off, and makes it very strong.
I did this over a year ago and it's never broken the wing mount, rear shock tower, or any other issue of that ilk, probably because the wing mount itself is very soft and tends to bend rather than transmit forces to another part of the rear end.
If anyone wants a picture of the wing mount off the car, let me know.
Car is for
sale so it'll not be long before it goes...