In Australia rear bags that sit on a rack and have a two layer panel on the vertical back section of the bag that allows the bag to slip over a hoop are quite popular.
Two of the well known brand names are "Gearsack" and "Ventura". In my case I have used a Ventura version for many years on all of my bikes as I am not a fan of the rigid rear top box style of luggage. In the pictures of mine below please note that it was originally black but with many years in the Aussie sunshine it has faded to brown.
The rack mounted on my K1100.
Note that the bag can be slipped over the loop facing rearwards or facing forwards and sit on the passenger seat if no pillion is on board.
When purchased new the rack system comes with a low bar which can be fitted in place of the high hoop just by loosening a couple of grub screws, withdrawing the hoop and fitting the low bar, as in the photo below, in situations where you will not be using the bag.
Here is a photo of the system with bag on my K75. I have camping gear strapped onto the pillion seat in front of the hoop.
and here is a photo of the bag in use on my K1100 on a frosty morning with the bag on the front side of the hoop resting on the pillion seat.
The Gearsack brand has a rectangular shaped hoop and the bags are a rectangular cubic style, but the idea is the same. I see from adds on the internet that a number of other brands now utilise similar styles and that the Ventura brand also have a range of different shaped bags these days. My bag is 25 years old and still going strong.
I have also fabricated a couple of hoops and mounting brackets for friends to fit bikes that the manufacturers don't provide a specific fit for. It is not a difficult fabrication job.
Hope this is of interest.
Cheers