I didn't know this BMW rear bag existed - maybe you didn't either:
https://www.shopbmwmotorcycles.com/oem-parts/bmw-motorrad-luggage-carrier-bag-77498562634https://www.ebay.co.uk/itm/394371024545It's main attractions, to me at least, are:
- it's a tidder bag, big enough for tools, small enough not to disturb the handling at speed
- it has a sleeve underneath to slide onto the luggage rack, plus two securing straps so its very secure
- properly waterproof
- carrrying handle if you want to take it away
And the downsides:
- price is north of £100
- seems quite rare (it's manual is dated 2016, so possibly out of production)
I have not tested this properly, by the way - just saying it exists somewhere in my house, and Christmas is coming.
I also have a QBag, and really wanted to like it, but gave up after a few months because I couldn't get the flimsy baseplate to stay still on the rack - the bag was always moving around a few millimetres however tight the straps were.
QBag link:
https://www.sportsbikeshop.co.uk/motorcycle_parts/content_prod/313404QBag pros:
- price is excellent
- fit is nice and tidy on a K rear rack
QBag cons:
- the 'baseplate' is difficult to keep tightly strapped on a K rack
- not waterproof
- not as sturdy as the BMW bag