The F800 depot sells a direct bolt-on OEM replacement for the fluid reservoir for $50. The reservoir becomes attached directly to the master cylinder in the lever, just like on the 1200GS.. but as mentioned in the above post, there is a reason it's mounted as a bubble head and not attached solidly.
I let Touratech rip me off on their brake reservoir cover, it's pretty solid, no bubble head and the fluid level is visible from the rider's side.
ADV decals, patches & flag? Here