I recall someone here was looking for one of these. I will be taking this off mine as I won't be using a top box.
I had this custom made in 316 stainless, with eough ribs to take just about any box mounting plate easily and still be useable for soft bag mounting or other gear carrying. It fits onto the stock mounts without any modifications or issues. It has been drilled for an SW-Motech adaptor plate, there are a couple of chips in the satin black powder coat, easly touched up, you're not going to worry about corrosion with this.
It is super strong, I have lifted the rear wheel, just make sure your sub frame is up to it :augie
Posting here first and if not needed then general release, no reasonable offer (plus postage) refused.