Goldbird3 has come up with this very simple solution which fits between the Upper Body and Lower Body of the RMU.
You can download the STL files from the Prusa wesbite here. Just two extra screws are needed to fit the bracket to the wall of enclosure, The lug at the bottom of the lower body rests against the wall.
This looks to be a very good add-on to the Filament Buffer, further extending its flexibility.