I would'nt say 'developed' but I made up a plate from 6mm aluminium, shaped and drilled to suit the rack bolts, drilled and tapped to suit the top box mounting plate.
Luckily, I didn't have to think too hard about this. When I traded in my '10 GSA, an 'old' GS Vario topbox and rack went with it.
In exchange, I got a 'new' Vario top box with the '13 GS.
And I'm glad I did so. The new topbox looks fundamentally the same, but put them side-by-side and the new 'box is bigger. Much bigger.
In it's 'small' configuration, the new 'box is about as big as the old 'box in 'big' configuration.