I repaired my rear subframe 5 years ago, after refit I dropped rubber sheets from the bottom of the inner wheel arch to the under side of the subframe for the full length, this was simply attached with self tapping screws. I then ran a sheet of aluminimum (from an old caravan panel) across the full width, using springs to stop the rattle against the exhaust, using self cutting bolts to attach to the subframe. The two benefits are that there is no ingress from road spray and the exhaust box lasts much longer. As I said i did this 5 years ago and still no signs of any rust, and it protects the brake lines as well.
Hope this of some help.
