Ok, finally, done, I made some rear bumper spats. I used a pair of proper rear bumper splitters from Ebay I got cheap, brand new items, and the sides, well, can you tell what the are from the 1st picture? I used a heat gun to flatten and reshape it. I just needed something to finish off the rear lower line of the car, so it'd follow on from the Hamann skirts.
These are the before and after pics.