I use one of these (not metal I know!), just clip-on-and-off when needed and carry as a man bag
£47 here
https://www.sportsbikeshop.co.uk/motorcycle_parts/content_prod/274769
Great for map, water, sat-nav (when off bike), small compressor and a stop'n'Go puncture kit, and a spare pair of gloves etc... Use it for day-trip when panniers and top-box not needed...