I had one on my F800ST and it readily swallows a helmet, although not much more.
A tip if you do get one - never lubricate the mounting rubbers with any protectant. They need to grip the rack so that in say a large dip, the force as the bike comes out doesn’t distort the locking mechanism under the case inner floor and cause the locking tabs to release. If this happens, you might feel a tap on your back and see a large ‘bowling ball’ follow you down the road.
Yep - I did this and it happened to me. I was very fortunate that BMW replaced the case free of charge. The replacement was subject to some cracking of the base with hard touring use that I subjected it to (rough roads) but I was able to ‘plastic weld’ the cracks and reinforce the base (all under the inner floor). I don’t know if BMW has improved the design in the meantime.
View attachment 119093