NO tank bag "looks good". But you really need something on the R11S. I had a Cortec but the mounting ring on the gas cap was terrible and way too tall. Found a new Givi XS307 quick mount and bag on fleabay for half price and it's OK. But not pretty. It detaches and mounts in a blink. Has to on a R11S; no fuel range.
I found this pillion bag on amazon:
https://www.amazon.com/gp/product/B06XX1HCH4/ref=oh_aui_detailpage_o05_s00?ie=UTF8&psc=1
It came in several days ago. Seems very small but it will be ok for a rainsuit. I have the BMW side cases too but don't need them all the time. Was thinking about a backpack as supplemental storage. Dunno.
Let us know what you come up with, please.