I needed an 18mm to 20mm adapter for my F91w. I haven't tested it yet, but once I do, there will be pictures showing it works. I just shrunk the original a bit along one axis.
Edit: Had some issues in the slicer. Once I figured out those issues, it printed fine. The 18mm section was a bit too long, but after less than a minute of filing it fit fine.
18mm_to_20mm_lug_adapter.stl | 212.5KB |