I modified Givi hard bags to fit my commando. I bought the frames for a different bike, then bought some generic metal tubing the same size as the Givi frame tubes. I went to my welder friend's house with the bags, the frames, and the tubing. We cut the existing arms off leaving just the mounting hoop. I held the bags attached to the hoop up where I wanted them and my welder friend put the new arms where they needed to be... from the mount point I specified and the mounting hoop. He's really good and it came out pretty nice.
It's a leap of faith to do this kind of thing because the first thing you do is ruin the frame by cutting the arms off it, so you are committed at that point. If it doesn't work out for some reason then you have bags and no working frame.
Here's my progression:
Frames with original arms for some unknown model motorcycle
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Here's the destruction part where I cut the arms off the mounting hoop
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here's the bike with just the frames and the bags not mounted. The frames are painted black to be a little more invisible to blend in with the bike frame. It doesn't look too terrible...
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and here's the mounted hard bags on the bike.
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The point of going to all this trouble is to have lockable, waterproof, quick disconnect, hard bags. I'm not going to wear a backpack when I ride. I didn't want saddlebags either... I'm just picky that way. If I sport ride, I unclip them and go. If I go somewhere and need to carry some stuff I clip them on.. I don't know if the BMW bags will work or look good, but you should hold them up and see if the lines look ok... then do what you have to do to make it work if you deem it doable...