Insert a u-lock through your bike rack so the open end of the lock is above the rack and the closed end goes below but hits a leg of the rack or some ...
Secure the briefcase with bungees or used bike inner-tube or both. It is best if the bungees or inner-tube help pull the case up to reduce the stress...
The briefcase may hit you foot as you pedal, which is the most common problem with this strapping method. The bottom of the case is the part that hit...
Insert a u-lock through your bike rack so the open end of the lock is above the rack and the closed end goes below but hits a leg of the rack or some part of the rack that stops it from going all the way through. This only works on some racks. Attaching a plank of wood across the rack can also work if you don't have a u-lock.
I´ve make a mix between your idea and some others from here, to get a CYCLOCASE, using a Notebook case and elastic strips. I put some pictures in here to help other commuters to do your own cyclo-briefcase!
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