Knocked up this new tail tidy. Used aluminum checker plate, about 2mm thick. Cut and bent to fit. Welded a piece of angle on the inside. It uses all the existing mount points and bolts. Now all that's left is to drill a few holes. Mount LED indicators and plate lights. Not sure if I'll paint it, might just leave it. Brushed aluminum.
Not off the bike, but these pics might give you an idea of how mine goes together - three parts, wouldn't be surprised if it was an early version of the Werkes one on ebay, or a knock-off of it - a channel piece, that fits under the tray and up the sides, an endplate to carry the rego, and the little box piece to hold the light. The channel piece has an upturn at one end, for screwing to the undertray, and a downturn at the other, where the plate carrier is pop-rivetted on.
Mmm... I like the folded under channel design of your original fender. Like you said better protection underneath from flying road debris. It's just a thin plastic tray under there. A sheet of metal is a much better guard.
Added a small backing plate for the LED plate lights. Trying out some black LEDs but I have ordered some small silver LEDs with an Allen key fitting. Also fitted some rubber dust/weather strip between tail tidy and tail light.