It is frustrating that manufacturers don't provide better guidelines for this sort of thing.
The Morningstar TS60 has a generous heat sink for a its rating so I think there is likely to be a reasonably margin.
However keeping electronics
cool helps the life considerably.
relies on air convection rising vertically so try and keep the top and bottom reasonable clear. A few inches at least. Simple measures like holes in shelves can help, if the a clearance is marginal.
If you can do this I doubt you will need a fan. Measure the heatsink temp with an IR thermometer during operation to get some idea how the installation
is performing. Below 60 deg C is acceptable, but lower is better.
Another option, is to improve the passive cooling
by mounting the electronics
on some aluminium plate. This needs some thermal coupling to the heatsink to have any effect but with aluminium cased electronics it can occasional provide some addition cooling at low cost, with no power consumption
and complete reliability