I am not fan of hard luggage, but for commuting, top cases are undeniably practical. Since this addition is meant only for on road, we tried how largest top case we could find would fit on Oleg's 1090 Adventure R.
We decided to go for Kappa KVE58B K-venture top case. It is also sold for few $$$ more as Givi Trekker Outback 58ltr. As all Kappa/Givi top cases it is very well made. Aluminum, 1.5 mm thick, nice plastic edges and reinforcements on the inside. It uses standard Monokey mounting system.
Here is how it looks on 1090 Adventure R. Even it is massive, it fits nicely.
Top case is mounted on Perun moto KTM 1090/1190/1290 Top luggage rack, using Givi Givi E115F kit and 5 mm thick Givi rubber grommets. More info regarding installation can be found here. Installation is a breeze. Since there is no need for additional plastic mounting plate, top case sits lower than it would if it was mounted on Givi/Kappa rear rack plate. It also mounts a bit more forward, for mass centralization. Pillion room is not compromised.
As a reference, bike is equipped with OEM pannier racks.
Personally, I wouldn't go off road with it. But for commuting it is invaluable.