After Tracy's post and looking at the rear lights again, tried the simple approach.
Trimmed a small piece of thin brass plate into 2 L shaped pieces and used the original supports to drill the 2 holes needed to attach the rear lights. The 2 pieces fit in between the 2 clamps holding on the fender and are just under the fender support. (Too bad I don't have the scale diamond plate here)
Corresponding holes drilled in the fenders
Rear light screwed on
Both with the coloured glass. The coloured cover is not completely pushed home, still have to glue in the transparent piece and I think I have to colour the indicator part of it too. Have to check that one.
Will paint the brass black before the final install.
Here pictures of the front and back of the PistenKing LED boards they make for ScaleART.
I will probably drill another hole through the brass plate and fender to guide the wires inside the fender ... will see how that works out.
Only thing left is to figure out where to makel the hole in the fuel tank to get the bowden cable to the servo of the front differential that is supposed to go in there.