OEM flat setup has been ideal. Even levels to my givi case tops if you wanted to carry something really big. This has been my daily for over a year. Working on resto modding an old toyota truck.
One thing I want to do is measure the grab handle sleeves I punched out so my cases would still fit with the rear plate low. Cut out some steel sleeve spacers and lock tite it all down for good.
The oem spacers from the grab handles work but are not ideal. They don't hug the shank of the luggage rack bolts.
*still need a fix for the left side case not fully seating in the lock.