Some say the K1600GT BMW tank bag fits the bagger & GA without interfering with the swing of the bars. Never tried it myself, but maybe your dealer can help - IIRC they are around £190 new & genuinely waterproof if my GSLC bag is representative. Some just use a small handlebar bag for small items; I think Wunderlich do something appropriate, but other options are available.
Probably the best option comes from RKA in the US, where Richard & Kathy have set to work producing a whole range of bike specific gear for your every need. Bespoke doesn't come cheap though, along with import/carriage costs: -
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https://www.rka-luggage.com/A08 BMW/K1600 Bagger/K1600Bagger_ALL.html
Sometime items come up on the K1600 forum UK riders for sale section - are you a member?
www.k1600forum.com/forum/uk-riders/