The Altrider pillion luggage plate for the last model R1200GS had an interface for Givi Monokey top case system and the GSessories Tail Gunner case. The Wasserboxer version is not out yet but my enquiries with Altrider suggest it is expected in the US on 1 Aug and elsewhere in late August.
Yep your local dealer can supply one to fit your top box like your old fitment ,but as an when you'll get it who knows its on back order as with most LC fitments