I will keep updating this original post as I progress rather than loads of posts threwout the thread.
The first ones going to be plastic as its cheaper than sheet metal then when I have it spot on ill make it from sheet metal or fibreglass.
First things first, make a template of the space available.
I don't want to take the tail light off yet as its my everyday bike so to make a template of the tail light area I made it around the light.

Then cut a template of the wheel well area and the fairing area which where it will hook under.

And I have ended up with the general shape of the maximum space I have to work with

Next I'm going to measure the depth of the tail light so I know how far back I have to work with then include that into my template.
Gunna go hunting for some clay to work with to get the shape for the tail light and look at what tail light I want to use, I wanna give the undertray a kinda of standard look so no trick, custom, weird shaped tail lights, either dual round ones or a single rectangular thinner one but DEFFINETLY want LED.