Inspired by Docok's posting of Resolution's high brake light I came up with my MK2 version:
In the lowered position
In the raised position so it's visible over the suitcase
I think this looks better than my previous effort which had the steel plate, it also means that I can just slide it down (rather than undoing four fiddly nuts and bolts then replacing two of them) when no case is on the rack and still have a high level light.
I know it needs a bit of fine tuning to level it which I'll do when I replace the bolts with four of the same size.