I used to have a Krauser topcase combination
like this mounted on my K1300S, together with a Krauser K5 topcase
like this (47liters). Therefor, you first of all MUST have the special BMW- handgrips (think it's called like that in English ?). So what you need then ( = IF you have these special BMW "handgrips") is
(a) an adapter plate (K1300S/K1300R- specific) and fixed on top of it
(b) a Krauser (more generic) plate specific for the K4 or K5 cases of Krausers. I think both (a) and (b) are included in the first link.
I still have a spare part (b)
like this as I bought myself a new such part (to have a fresher one to mount on my VFR800F which I bought last year), should you decide to use a Krauser topcase. But then again, you'd still miss the (a) part I mentioned.
So IF I understand the Krauser website properly, you'd be travel-ready after spending 290€ : 120€ for the (a) and (b) combination, and 170€ for the K5 topcase.
In my opinion, the Krauser topcase solution was the most elegantly looking I found, years ago.