You didn't mention the size of your leather briefcase, so not sure if this is going to be large enough.
The Aerostich Parcel bag, at (14”×9”×26”) could possibly hold a standard size briefcase. Available in both, waterproof Cordura or Waxed cotton.
They make some of the finest motorcycle gear around and their customer service is top notch!
http://www.aerostich.com/bags/should...s/courier-bags