Haven't had any issues with my BF Pocket Rocket and regular panniers, but your briefcase looks a bit wider than most panniers. I would try strapping a dowel or metal rod horizontally to the rack support bars partway down from the rack top so it provides additional backing to the rear part of the briefcase and keeps it from swaying so much.

BTW, the 'underbag' may still fit with a different rack. I use mine on a Blackburn rear rack and it works ok after just a bit of trimming of a couple of the velcro fastening tabs.