So I just installed the 690 Duke rear fender extender by Pyramid-Plastics.co.uk yesterday. The adhesive is still curing.
I can tell you that it is not an exact fit, but is pretty darn close. The only point to note is the curvature of the two surfaces is not exactly the same but with clamps while drying, I’m hoping it’ll be fine.
Their part number is 079305. Shout out if you find anything better.