Got a bm Ali box on my 800 looks good, big enough to take helmet, or all the usual bits, waterproofs, sarnies, t shirt n shorts for sun!
One good thing is that it looks the right size compaired to bike size, rather than looking like a laden camel.
I Just need to sort out panniers now, looking at Wolfman soft waterproof's new range, as I feel the Ali ones look to big IMHO.
Good luck with whatever you choose, it's like all things personal choice, some people like 1200's
Got mine when I bough the bike, as ex demo with extras already fitted
I think you will also need the rear rack plate to fit.
Sorry can't be any more help, give your local BM Dealer a call
size is big and small enough closed down 25 Litres, expanded 35 Litres which is big enough to take a lid (Arai Tour X3 no problem)
bought and fitted when I got the bike at BMW code DFK72/4654F800GS top case and mounts at a Total of £271.60 + VAT
plus points:
Picked up by the BMW warranty, key coded to your ignition key, fast expanded up and down, quick release to remove (2 levers not accessable when locked) and Bone dry.
I like the Idea of when 35L is not needed you can ride with it reduced to 25L size.
Good luck with your choice, theres a wide range out there.