@daddylovesky correct, the garage that installed the tire carrier fabricated a metal plate for attaching the top part of the trail rack to the tire carriers light post. It's literally just a big flat rectangular piece of steel with a hole cut in it for the gas cap to fit through. It's plenty strong enough for the 81L AluBox and its contents. I wouldn't swing on it, but I usually grab ahold of it for support when I'm standing on the rear bumper to pop open the Ursa Minor.
I'm not sure what you mean by adjustable supports?
I'm not sure what you mean by adjustable supports?