Your measurements were spot on, thanks again! I was able to print some more prototypes and they fit inside the airbox and over all four carbs very well. I made the CAD files parametric, so it should be possible to make carb boots for the GS850, too. They seem to be hard to get as well. But I still have to make some adjustments to make them more "printable", 'cause it all went really sideways when I tried to print them in some other flexible material.
I was goin to post the current version here for now but the forum software doesn't seem allow .stl files. So I'll probably upload the files to thingiverse or printables once I'm happy with them. Btw, is there a "proper" technical term for carb boots, so people can find them?