The BMW cases are great for touring, but a bit overkill for carrying just a briefcase to work everyday. For me, side cases add width that makes it tricky to maneuver around certain obstacles to my sidewalk parking place. To my eye, the bike looks weirdly imbalanced with just the 49L top case attached. The 28L top case is better suited for holding a helmet than a briefcase.
For a briefcase-friendly commuter case, you can mount a hard-shell attaché case to the bike. I wanted extra room for a rain suit or other gear, so I opted for a “Heavy Duty Tool Case” from Cases by Source (http://www.casesbysource.com/product...ts-tuc-11402-p
, see Pic 1). This 25L case is sturdy, sleek, and can be locked by either a combination or a key. It ended up leaking rain around the trim, but that was fixed with a thin bead of silicone.
I made a rigid platform for the case by sandwiching 0.5” foam insulation between epoxy and fiberglass cloth (Pic 2). The four solid epoxy cutouts are for the mounting hardware. I painted the platform black and glued it to the bottom of the case. Mounting hardware is hammered and sawn from aluminum stock (Pic 3). Small bolts pass through the case bottom and platform to secure the mounting hardware to the bottom of the case. Eighth-inch Al plates distribute the load within the case (Pic 4). There are no modifications to the bike.
The mounting works similar to stock top cases: You lay the case on the luggage rack pushing forward to engage the front clamps (background in Pic 3). The rear clamps are bolted to an aluminum plate that floats on large horizontal bolts (foreground in Pic 3). To engage these clamps to the luggage rack, you reach under the case and slide the plate back. You then tighten two nuts on the large bolts to lock the case on the bike. To remove the case, you reverse the process.
Pic 5 and Pic 6 show the completed product. Pic 7 shows it accommodating a well-stuffed briefcase. I keep a wrench Velcroed in the case so that it is always available for attaching and removing the case. It takes about a minute to tighten/loosen the nuts to remove/attach the case. Not as convenient as stock top cases, but I only occasionally need to take the commuter case off –mostly just to swap in the 49L top case for touring or grocery shopping.