I am wondering if the baffle, rather than being a complete soffit, could be a large standalone baffle that surrounds the speaker to the floor and then 2 or 3 feet in all other directions.
You can build half walls or 3/4 walls and they will still improve things (the arger the wall the more ideal), however, you have to kill all the rear energy. Green Mountains use solid cabinets - so that is already a good start but you really want to enclose the speakers soffits with acoustic damping and then heavily brace your construction to make the wall solid and ensure it does act as a "sound board". In practice this means plenty of MDF and it means filling empty spaces (cavities) with wadding. The John Sayers website I linked to explains what is involved - I would not recommend it in a rental as you are talking significant expense and time.
Genelec sell soffit mount kits for their 8050A monitors - you can look up these too - but again it is not something suited to a rental property.
FWIW: Improvements from soffit mounting are definitely audible (why else would studios go to so much trouble) however it will not be night and day. 5 - 10% improvement max...