Only use the surrounding space in ways permitted by the manufacturer. Boxes can interfere with ventilation, create unstable storage or fall when the door is opened.
What the observation tells you
The top surface is not automatically a load-rated shelf. Items can also hide a nearby heat or moisture problem and make an appliance harder to access. A clearance measured at the front does not prove that the rear ventilation remains open.
A useful next step
Check the model's spacing and load guidance, and move unsuitable stored items to proper shelving. Do not place electrical equipment or liquid containers there merely because they fit. Mention overhead obstructions when arranging access for a visit.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.