Was thinking about this for long trips... bikers have similar space and weather challenges.
1. Add a mounting plate to a luggage rack e.g. https://www.amazon.co.uk/E251-Givi-Universal-Monokey-Plate/dp/B00FYOQB5A
2. Add box of your choice e.g. https://www.amazon.co.uk/Givi-OBK58A-Aluminium-Top-Case-Trekker-Outback/dp/B00L81XHF0

Concerns would be
A. Weight!
B. Bike boxes are not designed for lateral loads...

M3W5sp 2015, MSCC, MTWC, Oxon UK